It can be done with Xtal sets but not with 2.4gHz sets. 2.4gHz locks on to one frequency and locks out all others.
That's really a bad statement to make - as it's patently incorrect

2.4 has upto 80 frequencies available, and depending on the system, uses either ;
all of them - and moves between them all - Foobar, Jeti, MPX, the new JR system, and some others
half of them - and moves between them all - Spektrum DSMX
2 of them - and hops between the 2 'channels' - JR DSM2 and Spektrum DSM2
or 1 of them - the one you're thinking of, which just sticks to that 1 channel - early XPS and some of the cheap 'toy' sets.
The 'single channel' sets are restricted to 10mW ERP output, whereas the others are allowed 100mW ERP output
To answer the OP, YES it can be done, quite easily, as wibplus says - by binding both Rx at the same time and using the relevant Rx connectors to match the two 'models' set up on the Tx

Mark