Check out the company Lehle, they make a slew of routing pedals for almost any situation.
I have the Lehle Parallel L on my board that allows a few things to be done.
what it's main purpose is for is to take a single input, run it clean to the output, then send a separate effects loop with a send and return. You get to blend the mix and control send and return gain.
However, you can also just use the return with no send to be able to mix two inputs into the one output.
Or you can also use the send without the return to send the input to another amp. And even then you could run another input to the return and mix that with the original input.
Routing is fun, complicated, and full of possibilities. And this was just the pedal I got for my specific needs (blending guitar pedals into a clean bass signal). I'm sure one of their pedals does everything you need in one.
I don't have time to look into extensively, but I think the 1at3 SGoS Switcher can pull off everything you want with one pedal when you use the C->B jack for a loop that gets blended into the B output. So you would input your instrument, run A to your other amp, run B with some effects and C with other effects path and blend/mix that into the B output.
The lehle website is very helpful with good visualizations and thorough manuals.