Sure it's allowed. We've only done a split stay one time -- went from POR for 4 nights and then to Poly for 3 nights. It was a nice way to experience two different resorts. I wouldn't want to do a switch for any less than 3 nights, so your plan looks good to me. The switchover day can be a bit of a hassle because you have to check out by 11:00 and then can't get into your new place until 3:00, so you're without a place to nap in the early afternoon. We had a rental car and just moved our bags ourselves, but I understand from other Dis'ers that Disney will move your bags for you.
The one thing to be aware of is that, if you're doing a package, I think you'd have to have two separate packages -- one for each resort. That might make your ticket prices more expensive. If you're just doing room only reservations, there isn't a price issue.
