Hi,
I was trying out my setup ( using 2.12.99 also) in similar ways:
Ican record four programs (record using the webpage) on the same mux (using a dual tuner HDHomerun), so I cannot confirm your problem.
However, when trying, make sure the tv-adapter has finished its initial scan (look under config/tvadapters/General, "muxes awaiting initial scan").
I think the device might be occupied as long as the initial scan is not finished , thereby limiting the number of channels available (should only take a few minutes or less if it is not the first time; If the initial scan hangs at a specific mux, there is probably something wrong, so you should consider removing that mux)
Alex.
PS: Can you watch two or more channels on the same mux using the same device, or does this give a similar error ?