ABY switch and Running bass guitar and synth bass

Radial Bassbone is a longtime favorite of people who do stuff like that. It has one channel with no controls and another with volume and tone controls so you can balance out the second bass. There are others but that one immediately comes to mind and the price is reasonable.

 

Custom one with volume/tone knobs p-bass style? It should be fairly simple, I could draw it for you

 

Separate amplifiers so you can use both in the same song....

 

Trying to mix line and instrument levels, this is a job for a mixer, not an ABY box especially since there is no need for switching.
You need a real, active mixer. Nowadays for 50 bucks you get a devent mixing device. Something like an Alto ZMX.

 

This is a legit point of view. My problem with some of these cheap mixers is signal degradation, though, so I could be talked into spending a little more than $50.

 

a great point BTW, especially if the synth has stereo output.

I'd say however that it's possible that the synth is also instrument-level and/or Hi-Z.
and you will actually need a stereo DI to plug it in.

I'm not sure, how Alto works, but I have a ZEDi and the only time I plugged a synth in it, it appeared it only worked with the DI switch on (and only with unbalanced cable).

I have some other digital equipment that outputs on two jacks, like Roland electronic percussion or Boss looper. I never was able to use their output in line-level inputs (in different equipment) as it was too quiet for the line level.

 

Buy a Presonus audiobox or firrstufio mobile used for under $100

 

Can you have 2 channels in the mixer? I use bass and synth and play both in the same song sometimes so I get 2 channels on the board. That is the best way.

 

The LEHLE 3at1 is a solid option, for what you want. But it won't help with EQing.