You can't induce over two independent variables in the usual sense unless your proposition is defined over a grid of values and can be reduced step by step using a well-order (like lexicographical order).
So the correct approach is not to try a double induction, but rather to analyze and solve the equation for which pairs it holds.
Thanks for your explanation.
It was a fools Quest because I oversaw n^m=n^(m-1)+n
Which by Chance Looks like more
But 10 years after my failed university try I finally have the time for learning. Not Just solving as I am tasked...