(I think) Mies' visit to Taliesen is documented by Edgar Tafel in "Apprentice to Genius". There might be some photos. Positive banter, if I recall.

I don't know about this topic directly, but what I would do is hit the library and quickly search the indexes of the big biographies on these two, for mentions of the other. This would be quick (a couple of hours), and would let you cite actual scholarship if you need to (rather than saying you got it third-hand on a website.)

flw: less is more only if the more isnt very good
mies: batman called he wants his cape back

there was positive correspondance between the two, until wright realized what mies was doing. then it all fell apart.

why are you looking for negative banter? seems architects and designers are often look to dis their peers as opposed to supporting them and finding common ground.

Mies, FLW and God drinking in a bar.
God, "which one of you said I was your apprentice?"
(this might be in architectural graphic standarts)

Thanks for the help! I'm not sure i'm allowed to disclose what these quotes will be used for as of yet. Its something quirky and it is for work so its not my own personal project. If you're an avid golfer of sorts and live in Chicago you may come across it in the near future. Thanks again.