Here is my thoughts:
1) It is already easy enough to share wp_users across several WPMU installs or WP installs, so that part would be pretty simple (even using HyperDB)
2) Having two WPMU installs being able to write to the same BP tables might be a disaster, but it might work. Once again using something like HyperDB, but it seems as though BP has issues with tables not being in same database as main WPMU tables.
I am still playing with this myself on test installs, so if I learn anything new, I would be happy to report.
Trent
you should use one install of mu and then use multi site manager it uses just the one database
you can then use it like so to access members profiles
eg
http://hyrxx.com/members/Hyrxx/ (dot com)
http://hyrxx.tv/members/Hyrxx/ (dot tv)
@hyrxx
So you are suggesting wpmu on hyrxx.com
Buddypress installed there
What is on hyrxx.tv?
wpmu and buddypress are installed on hyrxx.com and hyrxx.tv is another domain added using the multisite manager which allows me to use the same installation but for multiple domains at the moment its blank but i am going to move my main blog there
my freinds blog is also hosted on my installation in this way
http://designflaw.co.cc
rather than having to use subdomains
Got it, makes sense
Are you not concerned about duplicate content penalties from Google re: SEO?