I have not done this personally - but I have heard of people doing it successfully. Like anything you co-own with a 'friend' - it really comes down to how well you understand and trust each other. I have a recollection that Jeff Schewe did just this with a P65+; perhaps he will add his thoughts.
A friend and I decided to buy a P65+ together and keep his P45+. The cost for him allowed him to keep his P45+ back instead of trading it in (a factor for him because he sometimes needs long exposure for architectural shots). I bought the 645 camera and lenses (which he really doesn't use) and we share the two backs.
There have been times when the P65+ was needed in two places at the same time...the way we did it was "book" the time on the back and if the other person needed it, he would have to rent. That's only happened a couple of time in the two years.
We've now recently upgraded to the IQ180 and we split the cost of the upgrade. Traded in the P65+ but also kept the P45+.
I think the key for us is the fact we ended up with two backs not sharing one single back. For a lot of shots, either the P45+ or the P65+ (now IQ180) would be fine.