Assuming a single star system, how would two or more moons appear in a night sky? Far apart or near, would they show similar states or could they be quite different, i.e. would they both have to close to full/half/sliver at the same time? This is complex, as they can be at very different distances from the planet. I have a planet with three moons...
Orbital mechanics aside (because that can get really complicated) ... heck if this is a more fantasy-oriented story, just make it all up. Just remember one basic rule, the farther away the moon is, the longer its orbit and cycle will be, and we hardcore SF folk won't scream
TOO loud.
But the good news is the phase has no relation to how far away from the planet its orbit is. It's determined solely by where it is in the sky in relation to the Sun.
Each moon is a ball (or sphere) in the sky. The bright side is where the Sun shines on it. If you know where the Sun is relative to each Moon, it's easy to figure out what part is lit by the Sun and what phase it's in. If the Sun has just set in the East and there's a moon in the Eastern sky, it will be a crescent (the Sun is farther away than each moons so when they're nearly in line the Sun is shining mostly on the opposite side of the moon). If there's a moon overhead, it will be at about half. If there's a moon in the West, it will be full.
Likewise, at midnight, a full moon will be overhead, and any moon(s) near the horizon will be at about half.
Regardless, if two or more moons are in the same part of the sky, they'll be in the same (or very close to the same) phase.
Tallent read my mind! Jupiter has 3 moons, or maybe 4, that are always in conjunction. That means, for every time moon A orbits around Jupiter, Moon B orbits twice, and Moon C orbits 4 times. Sorry if I'm kinda vague on the deatils . . . been a while since I took Astronomy classes!
Anyway, cool idea!
That's not conjunction, I think that's called harmonically locked orbits.