I am a hobbyist who has been playing bass & singing for years. At first, it was difficult, but with time (years) & practice, I got better at it (and I will never stop practicing). On bass you have to focus on locking the rhythm section down with both emotion, and accuracy. On vocals, you need to focus on singing on key, and with emotion (and remembering the words - Trick : Write out difficult lyrics BIG on a big sheet of paper and stick it on the stage in front of you ).
I emulate my heroes Paul Mc Cartney, Chris Squire, Sting, Peter Cetera, John Wetton, Greg Lake, Jason Scheff, and Mark King. In doing so, I have a goal, or a target to reach for in my playing & singing.
I think you have to practice the two things separately, and also together, as a separate thing. Vocal training is a big help, and pre-gig vocal exercises can help your voice really shine through. Bass training is a plus, and some finger exercises before gigs can also warm you up. Also, placing the amp' in such a way that you can hear what you are playing easily helps, as does ensuring you hear the key elements of the drums (especially the kick, hi-hats, and snare).
To me, some time with a metronome is a big help in helping to discern, and address timing fluctuations during a song/gig.
The best compliment is if people credit you as both a good bassist, and a good vocalist. Funny, but, at best, I think you should be able to perform as only a bassist, or only a vocalist, and do a good job. Also, I think nobody can truly call him/herself a singer if he/she cannot sing backup/harmonies. Singing harmonies should also be a goal.
The bottom line is that nothing can be achieved without dedicated practice, and that should NEVER stop.
