Transformers connected series at secondary side
As long as both secondaries are rated for the same current, that should work fine. If they have different current ratings, you will want to take no more current than the lower of the two. 

The main thing is to keep track of the polarities, i.e., the placement of the dots. With series secondaries, if you get one winding flipped, the secondaries will be out-of-phase and you won't get any current, assuming the secondary voltages are the same. If they aren't, the higher voltage secondary would drive current against the lower voltage one, but just keep the dots straight and this is not a concern.

The real hazard is when you try to put secondaries in parallel. Get the dots wrong on this setup, and the two secondaries become a short circuit. Much smoke will follow.