Don't think of a dual voice coil sub as having "two voice coils" - you don't use it that way, and technically it doesn't. It's just one voice coil, that happens to be tapped in the center to give you more wiring options.
The advantage of a DVC sub
isn't higher power handling - it's that it gives you the option of wiring it one of two ways, opening up more doors for amplifiers that are compatible with it.
That being said - a 400w sub is a 400w sub. That's 400w for the whole voice coil... or if you want to think of it this way (which I wouldn't personally, for risk of confusing yourself), 200w per each half of that voice coil. OK, call it "each voice coil".
It works like this:
If it is a dual 4 ohm voice coil, you can wire that to create either a 2 ohm or an 8 ohm speaker out of it.
If it is a dual 2 ohm voice coil, you can wire that to create either a 1 ohm or a 4 ohm speaker out of it.
Technically, you
could use a 2 channel amp, with one channel going to each voice coil - but why would you? It would involve a more expensive, less efficient, stereo, class A/B amp, to power one single, heavy and inefficient subwoofer - which doesn't make sense really.
Also, if it is a dual 4 ohm sub, you wouldn't be getting full power out of the stereo amp. If it were a dual 2 ohm sub, it would make more sense to wire the sub to 4 ohms and then run the amp bridged, emulating a mono amp - which removes any risk that one voice coil might receive power or a different signal than the other voice coil.
What impedence is the sub, and what is the stereo amp you are looking to buy - and why are you looking at a stereo amp to power a sub?