putting 15 vct transformer in series to get 30 vct output
Primaries are wired in parallel and secondaries in series. No problem there at all but, there is a small chance that the secondaries are out of phase and the output voltage will be zero - just like when you have a battery compartment for two series connected batteries and you reverse one of those batteries.

Be prepared to swap over the secondary winding of one of the transformers to make it work but, chances are it'll be fine.