yes I don't trust it either, seen too many issues with pats and the 708, even the maxisys. yes wait until it dies then program the spare, but you should not have any issues using both pcm, you don't have to program back and forth anything, both pcm will have the same key and secret key stored and there shouldn't be any issue swapping them, except one of them does not have the same vin number and probably a different part number and calibration number or tear tag number, and that could cause an issue. but don't try it just in case the tool won't work.
but if you were able to program the new pats transceiver you shouldn't have any problem programming pats keys i will think.