Oh my goodness no!!!!
Those are made to split one output into 2, to run one guitar into multiple amps. Not to combine 2 outputs into one.
Merging two electrical signals by just connecting them together is bad. This is equivalent to plugging 2 power amps into the same speaker.
You won't get sparks and smoke because the guitar outputs are very low powered even if they have preamps. But they will affect each other, especially if you mix a passive and an active bass.
2 voltage sources merged directly causes bad things to happen, they fight miserably. You need a buffer of some resistors to seperate the two voltages, allowing them to merge gradually, gracefully, peacefully.
with the low power involved you're unlikely to actually burn something out on either guitar, but the results will be unpredictable tone wise.
Randy