just about everyone that works with hardware greatly suggests not to do that. no, you won't break the system or burn the ram, but the stability concerns are to be looked at.
if you start experiencing seemingly random crashes or other stability issues of that kind, just be wary about the fact that you're using two unsynchronized ram bricks (also, usually when buying ram, manufacturers package multiple bricks together from the same batch just to make sure they are as synchronized as possible)...
cliffs: sure, go ahead and see what happens, if you get crashes, exchange the ram for synchronized bricks.