What do you mean by double stack?

A double stack is simply two single stacks. (A triple stack is three, etc. And no, we are not talking pancakes or hamburgers. :-)>)

Often, the topic of double stacks comes up in compiler design, where you use one stack for operands and the other stack for operators, during the processing of an infix expression into a postfix expression.

Do not confuse these "extra" stacks with the primary stack provided by your compiler for passing arguments and return details. These "extra" stacks are usually coded explicitly, rather than implicitly, though some libraries such as STL can make it easier to do so.