I've had it go like LBS-bass said above before, when for some reason it just fell into place suddenly. Normally, it's terribly difficult for me to sing while playing bass. Of course, it depends on the song and what the bass part is like. Some things are easy while others are impossible, or nearly so.
I forget what song it was, but one night I was trying to sing along with the guys on a song that would typically give me trouble when all of a sudden it was like the veil was pulled back and there I was doing it just right...and without practicing doing it beforehand. Lol, it felt like I had just evolved to another level of intelligence (seriously LOL, but that's what it felt like). Or maybe like the stream had been cleared of branches so it could flow. I dunno, it was a surprise, though.
I've thought about that a lot lately and about how that was possible so suddenly. It certainly wasn't a gain in intelligence because I still have trouble. It was an anomaly. But how, or why? Can it be repeated? Maybe.
It may be similar to how I can strum a guitar and sing. I can forget about the strumming and just sing for some reason. But when I play bass, not so. I have to work it out. Normally. But I think that it might be related to which end I'm looking from. Normally I'm looking from the bass part end with the bass part being my foundation. I THINK maybe the night I had no trouble just MIGHT have been the opposite. Somehow I was looking from the vocal end and letting it be my foundation. Then the bass part just came along naturally.
I can't say for sure. Just try it it and see what ya'll think.