The white is urates and is a solid form of urine. It's waste from the bloodstream processed by the kidneys. The brown is undigested food from the digestive tract/intestines. So while everything passes out through the vent, they are coming from two different places. So...you can have an impaction in the digestive system and still pass urates.

My tegus are on mulch and they burrow completely underneath so that I can't see them when they're sleeping unless a tail is sticking out. They usually won't hibernate (brumate) while they are eating because undigested food can rot in the gut. Their instinct is to stop eating a few days or a week before going into their burrow.