You need more than one voice coil to run subs in parallel, but not necessarily a DVC sub. 1 DVC sub = 2 voice coils, 2 SVC subs = 2 voice coils.
To run a sub in "parallel", the positive terminal from the amp runs to all the positive terminals on the subs, and the negative terminal from the amp runs to all the negative terminals on the subs. This divides the resistance between the subs.
You can also run the subs in "series" aka "daisy chain" by linking the pos from the amp to the pos on the first sub, and then the neg on that sub to the pos on the next sub, and then the neg on that sub to the neg on the amp. Running in series multiplies the resistance of the subs.
As for resistance, two 4 ohm subs in parallel would create a 2 ohm load, while two 4 ohm subs in series would create an 8 ohm load. The lower the ohm number, the more power the amp can put out.
Let me know what kind of amp you will be running and what subs you will be running and I can let you know the best way to run the subs.
1989 M3 2.5 - 265 hp
1991 318iS - gots rims