A question about using a dual secondary transformer as a center tapped transformer
Yes, a B-to-C connection would be the obvious thing to try first. After making that connection (and before you connect anything else to A and D), measure the voltage between A and D. If it's 12V, you're all set.

If it's a very low value, then one of the windings needs to be reversed. Connect B to D and use that as your center tap, and then A and C become your "end" connections.