a PE is not the same as an SE license...for that, you are looking at passing the PE first then an additional 5-10 years of work experience, then another 16 hour exam.  

Dual license is not really going to help you...Engineering knowledge will make you a better architect, but most people even with both licenses tend to specialize in one field or the other. As I said in a previous post, it takes a lot of time to become fluent in both professions, a lifetime.  You can focus on one, or be mediocre at both.

double the liabilities?

Are you doing it for the interest of it, or as a means to get more money or a better position?

I've seen a few with both - but never talked to anyone that stamped both; that just sounds like professional suicide.  

I believe a lot of states let you stamp structural drawings with a PE (you do need a SE in NY, CA, IL, MS in SE is usually required; while a PE only requires a BS+exp).

Simply put

pros: you understand both approaches to building design, potentially easier to move up in interdisciplinary firms.

cons: liability and associated liability insurance costs; hrs of continuing ed.

My Dad has both; Architect and Structural Engineer stamps.  It was extremely handy before he retired.  He mostly stamped the structural on the small projects like residential or the odd beam replacements or resizing. 

Also, it helped land work.  A lot of clients prefer that single point of contact and don't like all the sub-consultants..  In-house, you didn't have fees leaving the office, coordination was a no-brainer, and we weren't so limited by cya over-engineering; We got to choose how much we were willing to take on.  Insurance?  Um... nothing special there at all.  He could also argue with conservative engineers who couldn't figure out how to do it showing them why our way works and the calculations to boot. 

It's also a highly marketable thing... Look for smaller firms.  Nothing quite as annoying as paying a couple grand for a simplistic foundation on a tiny addition and some framing.  That's the sort of stuff that bleeds small firms because the local building department requires an engineers stamp (something architects used to be considered competent enough to do).  The ability to do it in-house is valued because it allows you to be competitive without hiring cheap because 'the other guy' must hire an engineering company and get that overhead along with it...  It also gives you access to the dark side (the engineering companies) who get annoyed that they have to hire an architect for a simple building because of the building department regulations...  So you can market to either side.

I also wouldn't start over on your education.  Look for an accredited A/E degree.  I know my school had one and the only difference was the 4th and 5th year where instead of competition studios, you took advanced structures.  Therefore a ton of your architectural credits will transfer for credit and you might even be able to part-time student the degree (lots of crossovers in the regular classes like arch history, energy, and so forth).

detroit mercy offers a 5 year m.arch/ struct engineering degree.  i only know of a few people who did it

sorry, i don't know how long the program is, i think it adds a year to the m.arch....