Join the groups "thamnophis" and "garter snakes" on Facebook if you want more activity, forums are pretty slow these days unfortunately.

The substrate doesn't matter too much, they aren't picky, they will burrow, they will also climb, they'll swim, garters do everything, pretty interesting snakes. Just make sure the substrate has a dry area and doesn't stay moist for any prolonged period of time.

And no, keeping them together won't stop them from mating.