Assuming you have a really good reason why you'd want to do that (and assuming x1 and x2 have integer values), you can do this by combining EVAL and SPRINTF.
x1 = 3;
x2 = 4;
newValue = 25;
eval(sprintf('x_%i_%i = newValue;',x1,x2));
If x1 and x2 are floats, it'll be trickier since a variable name cannot have dots in it, though it would still be possible as long as you replace the dots with something else.
However, I really have to ask: Are you sure that you want to do that? Because at the moment I cannot imagine an application where would want to create variable names you don't know beforehand, which in turn makes it very hard to write an efficient program.
EDIT
There are many useful ways to store your data in arrays. If you really don't want that, you may be interested in accessing data via key/value pairs in a MAP, a feature which is available in more recent versions of Matlab. Thus, your key would become sprintf('%i_%i',x1,x2), and the corresponding value would be whatever it is you want to store.
