Depends on if youre running it bridged to mono or not.
If you're running your sub amp bridged to mono, get 2+2 ohm dual coil subs and wire the coils in series with each other on each sub, then wire the two subs together in parallel and you'll come out at 2 ohms total.
If youre running two channels unbridged to the box (one sub per channel), get 4+4 ohm dual coil subs and wire the coils in parallel on each sub. This will put each sub at 2ohms.
This is the ohm ratings you can get from a single sub of either type:
2+2 dual coil: 4 ohms with the coils in series, 1 ohm with the coils in parallel (don't try 1ohm unless your amp is hella beefy).
4+4 dual coil: 8 ohms with the coils in series (waste of amplifier power), 2 ohms with the coils in parallel.
Hope that helps.