Evaluate function F=C1*C2, where C1 and C2 are from two different solution

Hii..

I want to evaluate a function F=C1*C2, where C1 and C2 are solved in two different solutions. C1 is solved in Solution1, which is a stationary solution, while C2 is solved in a time dependent solution, Solution2.

How do I evaluate this during post processing. One way is to define the function and evaluate it while I am solving Solution2, I think. But is there a way to evaluate this kind of function, even if it hasnt been defined a variable during the solution.

Thanks

Dibyadeep Paul