Not being able to help you with switching 2 TX channels to RX (as this is what you want if I get it right), just wished to ask if you saw a new Lime board with two LMS chips. Don’t remember its name, saw it somewhere on their pages. Don’t know how much it would cost though :). But at least technically it should be able to give 4xTX…
I just started to use 2 RX channels in parallel in our custom Windows app, so far looks pretty decent.