Tortoises make a LOT more waste than box turtles do.
A tortoise should NOT be on sand. not not not not. Please have a look at Melissa Kaplan's tortoise care sheets, here:
http://www.anapsid.org/mainchelonians.html
Hay is not a good idea- when hay gets damp, it mildews. This can cause illness. Live grass is fine, but hay is a very bad idea for tortoises and turtles. Hay should be offered at feeding, but not left to sit in the habitat.
Repetit- you can help prolong the time between changes by scooping out poop and urine, but eventually you will have to change all the substrate. In the case of a tortoise kept in a rubbermaid tub, that will probably need to be every couple of weeks.
Jen, sand is also not good for box turtles. Mulch is better. I've been using cypress mulch for almost a decade now.
Also, I have NEVER seen a box turtle create urate crystals. That is very weird. My boxies pee copious amounts of liquid urine, and also make feces. The feces are usually in the water dish. Are you soaking your turtles every day to make sure they get enough hydration? It is not actually normal for a turtle to make urate crystals instead of urine. I saw an elderly and unhealthy boxie who would sometimes pass white material mixed with her feces, which may have been urea, but never have I seen another boxie make urate.
I am just curious- why do you keep your boxies separate? They are very social critters.
I used to be a reptile expert. Now I'm just an old turtle lover.