Bi-preamping...

[The 0x]

I just realized with my Bassman 135 head, that I could use both channels, and run full bass on the bass channel, and full mids and treble on the guitar channel, which breaks up easier. Has anybody done this before?

This is what my head looks like, except mine has black grillecloth instead of silver:
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[BillyB_from_LZ]

With only one master volume control, the channel that you wicked up the most would dominate the overall tone, there would be no real way to balance them.

Your best bet for some overdrive would be to plug your bass into the bass channel, run an instrument cable from the other input jack on the bass channel to a preamp or overdrive pedal, then plug the output of that into the normal channel. The Volume control in a Fender preamp comes after the first tube stage (and tone stack) so you could overdrive the first stage and dial back the volume with the volume control.