Well. I almost want to wait and see where this one careens off to before I run in and jack it up worse. What the ham-biskit, I worked all night...
Okay... I
think I know what you mean. You want to isolate (hear)
only one pickup, either the bridge or neck, discreetly, but in both ears at once. If that's it, then yes, there are
switchable stereo/mono headphones made. I have a crappy set I use at work to locate bad modules on membrane filters. Literally, to amplify a piezo signal... kind of an electronic stethoscope. The monaural image makes it easier to pinpoint the exact location of the leak. The sound quality couldn't matter less, so a super-cheap set of cans is what I got. I bet some outfit on this warm, wet world makes some that don't suck.
I think it'll work on a stereo jack on a Series bass, but I can't say I ever thought to try exactly what you want to do here. Headphones, yeah. If it's very important to you, buy an el-cheapo set first to test, then, if this is a feature you just can't do without buy some good ones. That's all I got.
I miss my bed.