I found in practise that I can talk or sing and play when the difficulty of given task is low enough that it doesn't use too much of my brain. If it does use too much brain, I just have to learn the part well enough so I will not have to think about it.
As everything, this can be practised. I'd start with:
1) when playing scales - think the
names of notes, later say them as you play them, later sing them as you play them (this excersise is very handy from other musical perspectives as well).
2) learn to
play bass and sing a song. Find song easy enough. Rock songs like
Smells Like Teen Spirit, where the bass is just thumping eights, are a good start.
Stand by me is hard rhythmically, but doable with some level of ability and analysis, and very nice song on bass+vox.
3) learn to
play and sing backing vocals in your band. This is usually way harder than singing the lead, and I deeply respect everyone I see doing it well.
By these excersises you learn how to use your brain for two less or more different processes at a time; talking is not that hard affter some practise. It's easy for someone, harder for somebody else, but available to the willing. However, keep in mind what is the goal. I don't think anybody ever invented some genius talk while playing reasonably well.