There is about a 3% chance that two, old style, grey transmitters will interfere with each other when you turn them on. Preventing that interference is what the new Shearwater Swift transmitters do. They detect that interference and change their timing to avoid it.
If you have 2 old style, grey transmitters that you are using at the same time, and you happen to have issues with interference, turn one off (by turning off its tank and depressurizing its line), then turn it back on. Each time you turn it on, it will have a new 3% chance of interfering.
I think you have to depressurize the line and leave it off for some few number of minutes before the transmitter actually powers itself off, and I'm not sure exactly how long that is.
I have used 2 old style grey transmitters together before, when diving sidemount. I have never experienced any issue of interference. But, I have not done that much diving in sidemount like that.
The main thing I would avoid is buying old style transmitters that are not actually Shearwater branded. If it is Shearwater branded, it will be covered by Shearwater's warranty. If it is branded Aqualung or Oceanic or whatever, then it would be under that manufacturer's warranty. Which is nothing, if it is purchased used. Shearwater's warranty is transferable and is 2 years, instead of 1 year.