I have a question that is a little bit of "engineering nature". Why can't I plug an audio signal(music from computer) and bass/guitar in the same speaker if I somehow wire outputs together without some device or mixer? I know that these two signals will be added to each other, but will it result in distorted signal or I could hear the difference between music and guitar? And what are 2 input AMPs made of if that is the case?
If you simply want to connect two sources to the same speaker, this is usually a big no-no. The problem is that the two sources will also be connected to each other, and they will try to drive each other, and neither one is meant to be driven by anything, so there is potential equipment damage.
However, if you are only connecting passive guitars or basses, then you should be fine. The pickups do not put out much power and will not be damaged by each other. You can wire two pickups in parallel on a Jazz bass, and there's no reason they must be on the same bass...
I wouldn't attempt doing this with a music source + an instrument.