I did a PhD in urban planning, and yes, I thoroughly enjoyed it.  But I did it several years after licensure, so my situation is not the same as yours.  My advice is to do one at a time.  Since you're already on the IDP track, keep going and focus on your license.  Grad school will always be there, but those IDP credits have a limited shelf life.

Unless you're superhuman, trying to do both simultaneously is a bad idea, in my opinion.

Why do you want to get your PhD?  Do you want to change careers, or 'enhance' your existing architecture career?  Are you trying to fill a void, or do you plan on going into academia?

If I were in your position, all options were equal, and wanted to make a clean break to change careers, I'd finish out the IDP and then go to school.  If you can find work in a firm that deals with the type of work you'd hope to do after the PhD, all the better.

Being licensed as an architect could actually help you in school, and would certainly help if you did anything construction-related after school.  This goes doubly so because a lot of what you deal with in architecture school doesn't really relate to the actual practice of architecture, and planning itself doesn't really relate all that much to architecture (school maybe even less so).  Having some practical experience (and proof of said experience) would maybe give you a bit more credibility as a 'planner-architect' as opposed to just a 'planner who used to be an architect'.

My own story: I am at the end of a second master's degree in a specialty that is tangentially related to architecture (maybe a bit more than planning is).    I chose to do this because I felt that my architectural education and experience did not reflect the way I wanted to work ('more style than substance' is the short version, not to denigrate either), and ultimately I see myself working in both a scientific and an architectural way because I feel that these two reinforce each other.  I didn't have enough scientific training (or the credential to show that I had this training), and so I 'filled the void', so to speak.  I'm not sure yet if this is a complete career break or just a transition, but it's an advancement either way.

My original intent was to finish IDP first, but the opportunities aligned in such a way that I went back sooner rather than later.  When I went back, I had most of my IDP done, but not all of it.  This means that, at some point in the future, I need to work for a registered architect for a little while to finish the license.  This is slightly inconvenient and limiting because it disallows a permanent 'clean break' with traditional architecture practice while still getting licensed.

The upside is that it's not so much IDP that I need to put away my knowledge from my second degree for several years to finish.  Plus, I still see myself fundamentally as a kind of architect, and am not entirely convinced that such a break is for the best; I believe that the way I work can be very useful for a traditional architectural practice, so it's not like the knowledge is completely irrelevant.

If you are just at the beginning of the IDP process, you might find that your priorities have changed by the end, and you may wind up changing your mind about grad school; alternately, you could reinforce your belief that it is the right choice for you somewhere along the way, and decide that you can finish your IDP later.  The nice thing about architecture is that it is a long-haul career, and so a 'wrong' choice may not be as damaging as if you were in something with a shorter shelf-life like, say, software engineering.

at a minimum i would recommend finishing IDP and at least a few of the exams before starting a phd program... you can finish the exams during winter and spring breaks if you want to (this is what i did)... it will be damn near impossible to work and successfully do a phd program at the same time... plus, if you're admitted into a program that provided funding, your funding agreement will likely forbid you from working...

Question to you eggheads... Do advanced degrees in urban planning ever make a difference in the actual field? Seems like all the good planning ideas happened at least 60 years ago and it's been a running gag of suburban disasters ever since. I know a lot of the planning process is political BS, but too many planners have thrown in the towel and only whip out the most mundane SPA schemes. If you are holding an advanced degree, do other decision makers stop and take notice? "Shit, this guy eats cobblestones for breakfast, we better listen." Even if it comes from an overzealous HOA.

I know I'm oversimplifying, but urban planning seems like so much fun on paper. Less so in practice...

@mixmasterfestus -- your advice is very helpful! Certainly when I finish school, I want to hit the ground running.

The reason I want a PhD is purely because of my own interests, I want to continue studying an aspect of architecture that has been usurped by planning (that I think needs to return to architecture). Down the line, I do want to teach and work, so the PhD can be postponed, but I feel so anxious to get back to school. However, I do not want to be the "planner who used to be an architect," because that isn't my interest, I want to be the "architect that dabbles in planning."

So it sounds as though I should dive into IDP and finish it ASAP, so I can get back to school!!! Thanks for all the wonderful tips!

Fun on paper, differently so in practice, let's say.  And, yes, plenty of BS (excrement, not degree).   Am crazy late for deadlines (planning, teaching) at the moment, will try to respond adequately later...

Oh, and several of my colleagues in the doctoral program for planning were architects, perhaps not surprisingly.

what philip said.

@rusty, professionally no one cares that my  phd is in planning except the university where i am teaching.  cllients have no point of reference for what a phd means and so far we have not been awarded large planning job to redesign a country or anything. 

in lots of ways phd is too specific to be useful in planning (as profession).  i am an expert on suburbia and the compact city according to my phd dissertation.

but outside of traditional design practice the degree has different value. 

ie, my research now is on building resiliency and adapting to climate change, that sort of thing.  it is useful because we just saw a huge part of the country get wiped out by a tsunami and there is going to be an earthquake in tokyo some day soon.  so we need to plan for this sort of thing.  in this case an academic degree means something, but really how often does the world bend itself this way?

it also helps with credibility in the arena i am working in.  ie, i am running a project with the united nations that covers several countries in asia and need to go deliver speeches and shit, so again the phd means they get to introduce me to the audience as Dr. jump.  it probably shouldn't matter but it does.  in 20 years after i have actual accomplishments it won't matter so much but since i am a young guy still the dr. part really does matter.

if you are planning to do reglar planning though am not sure what the point of a phd would be...