You probably need to turn down the level of the signal on your Kbd. I have an old analog modular synth and do the same.
The 2TR input is meant for comparison with commercial or other material as qbury mentions in the 2nd post - this is what it’s primarily for - for people who like to mix this way. Also once you mixdown and burn to a CD, it lets you listen to it over the original soundsystem. It wasn’t designed to be a 7/8 input source.