in fact I haven’t done many tests with multiple modulators as it tends to be impractical somehow due to the necessary mixing of sources. This will in general lead to a large peak to average ratio on the resulting RF signal and a better option is to use several transmitters and mix them in RF with a combiner or diplexer.
In fact this mxing might be the cause because it creates a single point running in one thread where the samples of all other threads must be collected one by one. Depending on how the O/S sees that situation it may decide to put all threads on the same CPU. Of course that’s just a guess.
For all these reasons the program issues a warning message that you probably noticed when you instantiate another Tx channel.
On Rx you don’t have the issue because the samples are distributed in chunks in turn to all demodulators. Perhaps on Tx side the mixing stage could prefetch samples from the modulators. This doesn’t change the peak/average issue in RF though.
Best regards, Edouard.