why does the sound change with the same audio on 2 tracks in multitrack?

It's perfectly normal dB addition. Two identical tracks, both playing at the same time, you get a 3dB boost.

In general, the more tracks of anything that you add into a mix, the louder it will get - although if the tracks are different, the build-up will be a little slower.

The solution is simple - it's what the mixer is there for. You adjust the levels of all the tracks so that they sound balanced, but the end result doesn't overload. Due to the way that the background processing works in Audition, it's just about impossible to overload the system with too much level (it's a Floating Point system designed specifically not to), but if you let levels get too high, you will overload your sound device, and things will sound rather distorted. So the answer is to make sure that the levels shown in the master section don't hit 0dB at the loudest point - then you'll be fine.