The only option you have would be option #2 from above. You can't have two transmitters transmitting to the same receiver. It just won't work. You would need two separate TXs and two separate RXs.
I think it was IAmLoser who posted the idea of using the two transmitters with one receiver as sort of a ghetto mixer... set the beltpack transmitter to one channel, and the plug-in transmitter to another channel, and then change the receiver channel on the fly to let you switch between the two wireless transmitters... haven't tried it, but it's one of those suggestions to keep tucked away for the day when you may need to do something like that.
I'm sure you could do that, but that would mean that the people holding the mic's would be controlling the mix, vs. the camera op who could control the mix by changing one button on the receiver.
Anyway, it's not my technique (I've never tried it) so I don't know all the ramifications; hopefully IAL will answer any questions.