Potential for interference, heat problems in that the one below will heat the one above (which I'm sure is a line from Sweeney Todd!), and the fact that the one on top won't be as solidly 'planted' as the one on the bottom.
In general terms it's best not to stack hi-fi equipment; in terms of amplifiers that's doubly true.
Oh, and the combined weight of the two is 28kg, which brings us back to the original question.