Are you trying to decide which of the two receivers to use or trying to use them both?
For the one receiver that has a single sub pre-out, I'm sure you know you can just connect that to the sub line in and be done.
For the 2 channel receiver with both L and R sub pre-outs, you could likely use just one of them. In general you don't want to combine two outputs into one (1 into two is fine) but since we are talking about a sub signal you probably can use a Y connector to combine the two outputs into one for the subwoofer because the bass is likely going to be the same in both channels. Try it both ways - you can't hurt anything.
If you are trying to use both receivers, with the 2 channel receiver driving the sub, then that will be complicated. You would have to split the sub pre-out from the main receiver with a Y connector and feed it to a line level input on the 2 channel receiver (aux, cd, tape, etc). But then you are back to how to send the 2 sub pre-outs to the one subwoofer line in as above.