Fulfilling Your Role
One of the most common problems that guitarists run in to within the typical two guitarist lineup is the almost inevitable clashing of sounds as both play similar things. This is typically a problem unique to guitarists, as no two other spots in a band are consistently at odds with one another within the mix. Below is a breakdown of what I think are the three key components to a successful two-guitar dynamic.
Communication
As with just about everything, communication is key when working with other musicians. With that in mind, it's generally a good idea to contact anyone you will be working with ahead of time with any questions you have for the upcoming set. Never hesitate to message another guitarist to work out lead/rhythm roles or talk through parts and chord inversions as the clarification will only lead to a smoother rehearsal. The more you know about a bandmate's thought process, the smoother the set will go.
Expectation
In general, most worship leaders expect you to come in with your guitar parts learned and practiced beforehand in order to ensure a smooth rehearsal. Depending on the leader, you may have some creative space to make parts your own, but remember that over all, the set was planned with you playing the recorded parts in mind. I often have to humble myself and remember that the original artist has sold hundreds of thousands more records than I have, so it's generally better to play what they recorded than work out something that I think is cooler. In the same vein that you wouldn't be down with a drummer playing a cool beat in 5/4 or a bassist ripping a Victor Wooten line during a down chorus, you are expected to play parts that edify the overall band sound.
Fulfillment
When actually rehearsing with the group for the first time, stay aware of the band sound and work to fill in any gaps in the overall sound. Ideally, one guitarist should be working to create a body for the song while the other adds leads. If your job is to do the latter, be sure to support the melodic flow of the song and stay out of the way of the vocals. Remember: face melting leads are cool, but vocals are always king. If you are working within a secondary guitar role, try look for every opportunity to add to a band's fullness without getting in the way of what everyone else is doing. Guitarists who thrive within these parameters often know how to play differing inversions than their counterpart as well as when it may be necessary (or unnecessary) to play swells or other ambient things.
An important trait that I've noticed in mature musicians is their ability to sit back and give a song space. I firmly believe that sometimes the best notes for a guitarist are rests, both from musical and spiritual standpoints. Aside from clearing up the overall mix, taking measures off will inevitably add emphasis to the parts you play when you come back in to a song while also creating a better dynamic shift for the song as a whole. Next time your instrument doesn't really have a part that adds to a song, take a few notes off and sit on whatever lyrics are being sung. I'm convinced that some of the best lyrics in worship music occur over measures that do not have any guitar parts.
Cheers,
Ryan