I'd say your best bet would be to test things out a bit, I'm currently also researching into it as at the moment the 5 box is a bit out of the question, so I'll start 3 boxing (still need to pick up 3rd account though, gosh I'm lazy).

I have a lvl 70 Feral Druid, and from what I've read so far, 2 boxing instances with a lvl 70 sucks xp-wise.

Once I get my third account up and running I'll try and run some tests and come back here with more info.