From the little I know about antennas, receiving of two antennas side by side is not an issue.
But you can run into problems when one of them starts sending, because it couples in on the other one, and will thereby decrease range of the passive one.
I will talk to one of my colleagues who is our antenna/wireless-guy at work later, what he thinks about its. maybe he can do some measurements i he has the time to.
From my personal experience with the PA-unit, I did not recognize any problems. Indoors I have a range of 20-30 NM on the ADS-B with no dropouts, but I can't tell what the P3i side does, because there has not been any othe P3i device in range so far.