Really?!?
If the coverage footprints of the two repeaters generate that much interference based upon the ability of the signals on the inputs, with or without PLs, to cause interference to the other repeaters, you have a problem that adding a PL will not solve!
Let us not forget that with two repeaters on the same frequency with massive overlapping coverage; once the squelch is opened by the signal with the proper PL, it will "hear" the other transmissions on the same input frequency, subject only to the "capture effect" for FM.