I've never kept garter snakes but I'd be afraid to house them together if there's much of a size difference. Even snakes that don't normally prey on other snakes
may sometimes just "be predators" & even kill a mate* (instead of breeding) in a captive situation. (*seen it with a pair of c.b. amel "black" rat snakes) I've seen
garters feeding, & like most other snakes, they get really wound up going after motion.