I wouldn't say that all coasters are designed not to stack...
Magnum XL-200 will always stack one of three trains in normal operation because B (gravity) is longer than C (transfer table). Top Gun at PKI will always stack because there are only three blocks on the ride and two trains, so the second train has to be on the block brake before the first can dispatch, and the first has to be a trainlength or two up the lift before the second can enter the station.
Some rides, on the other hand, should never stack. Gemini for instance is designed to never stack even with six trains running. It's all in the block timing.
--Dave Althoff, Jr.