WHen I was a kid, I had two transformers from my slot racing set. Basically, I looked at them as being two batteries that I hooked up in series. Ran them for quite some time that way and, yes, the voltage added.
Assuming that the secondaries are isolated from ground, you should be able to hook them up in series just as if they are batteries hooked in series.
HOWEVER, if the secondaries are hooked to ground for some reason this could become problematic. However, if you break any grounding link on the DC side, you should then be ablt to daisychain them.
The ground link elimination is not unlike that which must be done on the AT/ATX power supplies when you daisy chain them.