I have a Pro-88 too. While I don't have the same setup you think about, I do have at least three other USBs with MIDI controllers going at the same time without issues.
You might want to consider a powered hub to be on the safe side regarding power consumption. Also, you don't really need another Pro-88: I'd go for one of the other weighted M-Audio keys without the cost added by all the controllers.
Ciao, Francesco (francescohp.com)
Being in a band is like being 16 all your life.