do you think that's what ppl like geddy lee, tom araya, sting, les claypool, lemmy do? make the bass part second nature? i'm kind of curious if that's what they do or they can just pull that out of their minds lol.
Compare it to playing drums.
You start with a steady beat, bass on the 1 and 3, snare on the 2 and 4 and the hi hats playing 8ths.
You do that until you feel comfortable. Then you take one of the three things you're playing and make the pattern a bit more complicated. You struggle with that bassdrum beat that falls between the hihat strokes, your arm wants to hit, too.
Once you got that sorted, you try the next one, playing more notes on the snare - same struggle.
Then the hats, same same.
Then you start varying the patterns for each bit. After a lot of practice, this starts working.
As you progress along, things that you wanted to play in the beginning, but never managed to, become easy.
When you come across a complex beat you want to play, you don't have to go back to the roots and build that beat from scratch any more, you more or less just sit down and play it after a couple of tries.
Geddy, Sting and Les struggled when they tried to sind and play bass for the first time - as everybody else did.
When they, after a lot of practice, managed to seperate bass and voice in their brains, the whole thing became a lot easier.
Nonetheless, in my experience one of the two should always be well within the comfort zone of one's abilities. When you're playing bass at your technical limit and try to sing something very challenging for your voice, things get hard.
Keep one of the two at an easy pace and you can push the other no problem.