There is no such thing as a "DC transformer". The physics of transformers requires a changing magnetic field to transfer power from the input to the output. The details of your question therefore make no sense.
However, you can simulate a center-tapped transformer by using two identical transformers. These are of course real transformers that run on AC. In this case you want two transformers with 6 V secondaries. Connect the primaries in parallel, and the secondaries in series. The node joining one end of each of the secondaries will be ground, and the other ends of each secondary one of the 6 V AC power points.
The only thing to watch out for is that the transformers be connected with the right polarity. Flipping either the primary or secondary of a transformer flips the output by 180°, which essentially negates it. A real center-tapped secondary drives the opposite ends out of phase, so that's how you need to arrange the two transformers. The easiest way is probably to wire them one way and see what you get. When wired correctly, you will get about 12 V AC end to end. When wired the other way, you will get close to 0 end to end, but still the 6 V AC from each side. If you happen to get it wrong first try, then flip either the primary or secondary of one of the transformers.