^^ wot he said.
I've got four tuners on my system - two the same, then one each of two other brands. The channels can be merged so I can pick whichever tuner is free, even across DVB-T(2) and DVB-S2. You need to think about how tvheadend works - networks, muxes, services, channels and tuners - they're not 1:1 mapped.
USB tuners can be a challenge because of the way in which Linux assigns numbers on boot - simply, it means they can move around. Using the same tuner type actually works in your favour here, as it doesn't matter whether identical tuners are ABC, ACB, BAC, BCA, CAB or CBA. There are ways and means to hold the buggers in one place otherwise, though.