Which musical are you playing?
I've done several, am halfway through a run of Bye Bye Birdie right now, but never on keyboard, just on brass.
It's tricky, there's never enough room or light, the music has mistakes, there will be many confusing cuts added, etc. (It's still the single most fun thing I do in a year.)
It seems to me the keyboard parts are pretty obvious about which are really essential and which are nice-to-have extras. I assume you're covering mostly string parts while the piano player handles most of the workload. Given that, it isn't all that critical which you play. I would mostly play one book, and try to identify (maybe by listening to the soundtrack) where you might have to add from the other book, and do this by inserting photocopies as needed. I wouldn't assume you will have room for two stands. Pits are cramped, and everybody is in the same boat, no prima donnas, so any room you take is stolen from somebody else who needs it too. If you are new to this, proving you are an easy person to work with is really more important than demonstrating virtuoso keyboard skills. One will get you hired again, the other will not. Put tabs in your music to help page turns, copy as necessary to cover impossible ones.
And know the show. It is not always possible to follow the director (if you have one) or the beat, and actors screw up and blow lines, skip verses, etc. Some of it you have to just feel and follow.