Sorta hard to come through with that. Keyboards generally take place of a piano player. Keyboards are usually looked upon as an electric piano/workstation. It would be really hard to have a piano player AND keyboardist in a worship band.
Not at all. It's not that hard, but it largely depends on what styles you and the pianist play in. I'm guessing the pianist probably plays melody with chords, or, if he/she is more adventurous, plays extra little riffs, plays thicker/altered chords, and so on. Or, depending on the song, plays a more rhythmic groove.
The challenge of playing with another keyboard player isn't as complex as it might seem. Before you get there though, think a bit about how you'd play keyboards if you were the only person playing keys, because it's not that much different.
If you're the only keyboard player in a standard band with acoustic and lead guitar players, bass, drums, and a few vocalists, you can't and shouldn't just play whatever. Every musician should be thinking about what the others are doing and how their own playing fits into that (admittedly easier for some instruments, like bass and drums, and often overlooked by instruments like acoustic guitar when playing with a pianist). In a band setting, you don't want to step on other people's toes. You usually don't want to try and play the same part someone else is playing.
What you want to do is find spaces in the music and fill them. You don't want to fill *every* space; space is a good thing. But you don't want to overlap much with everyone else. If it's a climactic part of the song (an intense bridge, maybe) with a fairly high vocal part, you might have the acoustic guitarist playing fairly high on the fretboard and the lead guitarist playing fairly high riffs. If you also play really high, you're leaving a fairly empty midrange; that midrange is where you'd want to play. In addition to filling harmonic space, rhythmic space is important too; finding time when fewer people are playing. The first example that came to my mind was the verse on Hush by Deep Purple. Jon Lord on the B3 is doubling the guitar and bass rhythm while the vocalist is singing, but at the end of each phrase, he plays two chords a bit higher up that fill in the space.
Playing with a second keyboard player is different only because it cuts down the type of sound you can use (i.e. you wouldn't want to play an acoustic piano sound) and it cuts down the type of playing you can do (i.e. whatever the pianist is playing for a particular song).
On keyboards, you have a lot of other options that an acoustic piano doesn't have. You can do pads, you can do synth lead riffs (although, since it's for worship and not a rock show, you have to keep moderation in mind, and play well without distracting everyone from the real reason they're all singing), you can do more subtle synth riffs. If you have a keyboard with good organ sounds, you can add B3 chords, pads, or riffs. If you've got good electric piano sounds, you can add little counter-melodies or create a groove. There's a world of possibilities out there; it really comes down to how familiar you are with particular styles and instruments (it's better not to play organ riffs at all than to do them in a way which doesn't suit the organ and is distracting, for example).
Your one biggest asset will be your ears. Not only will you need to be good at listening to what everyone else is doing (honestly, this should be a skill that anyone making music in a band setting, or any setting where musicians aren't playing from notation, should have), but you also will want to listen to recorded music, be it worship or otherwise (personally, I lean towards otherwise; I'm not aware of anyone who plays keys for a worship band who's considered one of the big name keyboard players). If you don't have ideas of your own, you need to listen to what other keyboard players do and figure out how to adapt their playing to your music, and hopefully, in the process, become better at making up your own ideas that fit well with your band. The second-biggest asset will be your ability to improvise. You're not *ever* going to see notation, either because there won't be any or because the pianist will be playing from the notation and you don't want to copy her. Playing without notation is a *very* different thing on guitar than on keyboards.
So, definitely don't write this off; if your team has regular practices outside of Sunday morning, you might want to start there, playing keys in practice to get some ideas and see how you fit but sticking to guitar on Sundays until you're comfortable and your bandmates are comfortable with you on keys. Good luck!