If they both have polarized plugs, they are probably OK, but if not, you need to do this check.
Connect a wire between the common terminals of the transformers. Connect wires to the hot terminal of each transformer. Plug the transformers in and adjust them both to the same voltage (5 or 10 volts). Take the wires that are connected to the hot terminals and brush the free ends together. No spark means they are in phase. If you get a spark, unplug one of the transformers, reverse the plug and try again. Put a mark on the transformer plugs so the next time you plug them in they will be phased.
