Get to know one or the other, guitar or vocal parts, so well that you can do it in your sleep, then add the other. You should be able to do one or the other without thinking about it.
I was in a Praise band for several years playing bass and guitar. I sang harmonies as well as the melodies and it all got very easy with time.

Here's a fun game, find a song you know, learn the lead (melody) and play that while singing harmony, or the other way around! I did that with Amazing Grace and it was interesting to say the least!