FWIW, I used to have the same configuration (slightly different modules) using one transmitter and one light switch between house and detached shop. The 4 button transmitter should have no issue doing what you want, however there could be an issue with range. The further two devices are apart, the more the signal degrades. It's possible that the second transmitter is right at the limit. In any case, there should be no issue using more than one of those transmitters so your issue must be with the signal.
The best option would be to use a signal tester, but I don't know where you can get one these days. I have used my XTBM for similar tasks and located devices that were damaging the X10 signal which caused issues like you are experiencing. Without such a tester, the alternative is to turn off circuit breakers and unplug devices until the signal works. Once you isolate the culprit device, the solution is usually adding a filter to eliminate the problem.