IMO you should run:

back/bis
tris/chest
shoulder/legs

reason being if you hit your back on day 1 and then your bis on day 2 you bis did not fully recover from your back workout. as we all know back work hits bis just as hard (ie. pulldowns, rows)

same for tris and chest. if you work tris one day then chest the next your tris did not fully recover. tris are very involved w/ chest work.

so you do the pairs together to totally exhaust them then start the recuperation phase.