Generally I've found guitarists who play bass and write bass lines to be somewhat "odd" in the approach to writing bass lines. Local guy to me is a guitar player/drummer who plays some bass and has an approach to bass I find rather odd. I can't explain it properly, but he comes up with bass lines I never would have come up with. They aren't really bad, nor are they overly complex just way different from what I would have done over similar guitar progressions he was playing. Years ago I was in a project with a guy who wrote some cool rock originals. He just let the bass player (he had a few over the years) do whatever they wanted. He and I came up with some cool stuff, and frankly I think it's best to let the bass player come up with the bass line 90% of the time. Wish I was still playing with that dude.