2 Center Tapped Transformers together
Basically you can't.  If you connect the secondaries of your transformers in series, you will get 30V across them but the common connection will not be a center tap since one side will be at 18V and the other at 12V. If you got 2 of the 18V transformers, you would get a center tapped 36V transformer by connecting the secondaries in series.  Two of the 12v transformers connected in series will give you a 24V center tapped transformer.  You really need 2 15V transformers and they don't have to be center tapped. When you connect them in series, the common point becomes the center tap of the 30V transformer.