The following works for any math symbol which you want to put on top of one another.
The command \displaystyle_{command1}^{command2} does the job (where “command1” is what you want to place below and “command2” above). The disadvantage is it moves one symbol slightly downwards in such a way that the space between the two symbols is vertically centered.
Another possibility to solve the problem is the command \stackrel{Above}{Below}. Here, the symbol placed “below” will be vertically centered (e.g. putting a reference above an equality or equivalence symbol) with the size of the “above”-symbol slightly decreased and place right on top.
