This questions pops up a fair amount here, so a search will give you some more info.
In my case, I only recently started singing (over the past 5 years or so) and I have found that it really helps me to know either the singing part or the bass part very well 1st, and then add in the other. In most cases, I try to have the bass lines down completely, then I start to sing. I've found that if I have to really concentrate on both the words and bass line at the same time, it's usually a disaster
After you do it for a while it gets a whole lot easier. I'm to the point now where I can usually work out both the bass line and vocals at the same time, unless one of them is particularly difficult .....
