tvheadend supports multiple dvb cards, and can support cards with twin tuners if they can be independently used under linux. This means that each tuner has it's own /dev/adapterX entry and that the use of one tuner does not limit the function of the other tuner.
Already with one dvb-t adapter can tvheadend record more then one program at a time. The only limitation is that both programs must be on the same transponder. With 2 adapters you can record multiple programs from 2 transponders etc. So if one transponder has 4 tv stations on it, you can record 4 programs from all 4 stations at the same time.
There is no need to define which device tvheadend will use when you schedule a recording. If you have 2 dvb adapters that both can receive the same program, then tvheadend will determine itself which adapter it will use based on the availability.
The only limitations are the available CPU power if you need to decode, (USB) bandwidth to the device and how quickly your HDD can save the streams. But on modern hardware you never hit these limitations unless you try to record a hugh amount of programs at the same time. I have done tests with 3 cards and 15 programs on a simple amd 3800+ cpu using 250Gb IDE HDD without much problems.
Hein