A double stack works but you can do even more with a triple. Since I leave each tray layer in mine with just the hole they tend to collect on one level even when I've got more than 2 to a cage and only go to the other level when doing some sprinting laps, to get away from someone in a bad mood, or to chew on something I've put up there. If you took the floors out they would probably use the space much more evenly and benefit from triple stack cages but I play musical cages all the time to get breeding pairs together and babies weaned. I'm always switching out rectangle trays for cutout trays and back again. Just make sure you break up the space enough they can't fall too far if they miss a jump.
If they are already introduced to one cage a single water bottle is fine but a back up is always a good idea if it might go unnoticed how much is in the bottle from day to day. Do not use bowls unless that's what they were raised on and you need to convert them. They aren't always good about bouncing around the bowl and getting repeatedly wet will cause fur or skin problems. A glass lixit 16-32oz bottle is best. Many get the glass living world bottles because they are sold everywhere but they have a much shorter lifespan. Some of my lixits have seen 10 years while I just threw away another living world someone bought new and sent with a 2 year old chin. Some have ok luck with kaytee bottles if you completely ignore the directions and fill the bottle to the top with room temp water to establish the vacuum that valve bottles run on. If you don't do that and actually watch the max fill line they will leak for a long time before the vacuum forms. I also have some really old critter canteens and my newer ones the rabbits are using also work great but they don't make a glass version. We stuck one on a chin cage temporarily when the living world bottle failed and the chin stood on her hay rack, leaned across the door, and chewed a hole in the top of the bottle through the bars.
