The guys at the tire shop were stunned to find out another Tahoe/Suburban in the shop that same day I was getting tires mounted had the same fob code programmed!
They found out when the other truck was done. They moved it outside and locked it. Mine was still in the bay when the horn beeped and my technician got locked out. He unlocked my truck and the other truck beeped and unlocked before the other tech could walk away, so that guy locked the outside truck again. That locked the tech out of my truck a second time.
It was a minute or two before they figured out what was going on, so the lock/unlock battle between the two vehicles continued a few more times.
They said they have never had that happen since they started working there. "What are the odds of that!", they said...
We all had a good laugh!
Frequencies between two fobs from the same vintage should be the same. I believe they are actually transmitting unique (or in my case almost unique) codes to the receiver in the truck. I would think you should be able to program the same fob to two vehicles. That might be easier with a Tech 2. I have never tried what you are thinking of trying. I think it should work.
From what I have seen in programming instructions, I am pretty sure you need to start the programming process like this:
The fob you want to be using on both trucks would need to get added to the second truck it currently does not work on.
Programming on that second truck needs to be initiated with the fob that the second truck already has programmed.
You may need to get more guidance from people who have already done this. I am sure you are not the first to want the same fob to work on multiple vehicles.