Actually, it is possible to breed DT to DT, especially if they are not from the same parents/background and have no spinal deformities. It is really important to watch for deformities, but it can be done for a generation or two with proper culling if any issues do appear. A few might be DT depending on what the other fin type was, a more dominant trait fin type might overwhelm the double tail gene, but if you breed your F1 spawn back to the DT you would get some DT babies.
~Ask me about discounts on beautiful aquabid fish! ~