What Vchick said - there is a strand of silica in between the two coils, so altogether the Kanger BDC's have three wicks. The heads are the same whether you are using the PT3 mini or full size. They are also compatible with all of the aerotanks.
You can get a good picture of this in Bursardo's review of the aerotank mini and mega where he takes one apart. Jump to 31 minutes to see the Kanger BDC dissected:
https://www.youtube.com/watch?v=n6VbZwFcgCo
Note - there are two versions of Kanger's BDC. The one seen in the video is the new (v2) version of the Kanger BDC. Phil compares this to the single coil Kanger heads that are compatible with Protank 1 and Protank 2, but a better comparison would have been between the v1 and v2 BDC's. As far as the wicks go the v1 and v2 BDC's should be largely the same afaik.