There are a number of approaches to your problem.

Some times one solution is to take out one of the tracks all together or replace it with a piano part played on a piano with a different timbre or perhaps played in a different octave

or on a completely different instrument?

This approach would mean that you would not need to apply sound engineering to solve the problem as the problem wouldn't be there.

In terms of sound engineering, it is a common approach to try and fit each instrument into their own sonic space.