I remember back quite a few years ago when Linn dealers would never have more than one pair of speakers in a listening room. I don't know if this is a practice they still observe, but it's definitely a good idea.
Most dealers' showrooms feature multiple speaker pairs so customers can quickly audition different models of speakers. While convenient, it certainly isn't optimal.
There are several reasons for this. First, the woofers in a disconnected pair of speakers will absorb bass energy at various frequencies, changing the overall frequency response of the room.
Secondly, unless the dealer has really done their homework, the best position for one speaker isn't going to be the best place to set up another model.
The same can apply to your home audition process.
If you have the time and energy, do your best to remove the older speakers from the room while auditioning that new pair. Spend the time setting up the new pair to a place they are happiest with.
Whether home or dealer, one pair per room's a good idea.