Can the game be running for any% if i use Steve by yorself (solo, but 2 characters playing)?

Sure! You can use two players by yourself. You can connect second player when it needed.

can we add a filter (not category) for 1/2 char used then?

because they are both Any% but slightly different

I think it isn't need to mark uses two players

Maybe it's worth to do? Steve, as an additional character, can jump higher, which allows you to perform some glitches, as well as the technique and routing when playing for two characters is significantly changed.