Nomes, you may have been pulling off housing them both together, that doesn't mean it's what's best. I've seen people argue the same thing with their snakes, and lo and behold sooner or later, something bad happens. Cannibalism is not your only problem when housing different species together.
Different species carry different bacteria in their gut. Housing two snakes together creates an environment where there may be ingestion of the foreign bacteria from the other snake (the other species). This will cause an upset in its own bacterial fauna and can cause it to be ill.
As for garters being communal, well let's just say they're not living in a terrarium! Keeping snakes in captivity is NOT the same as one in the wild. Snakes in the wild don't have to come across their feces everytime they move around cos they're not confined as in a terrarium.
All feces aside, snakes in general, being housed together, makes it difficult to figure which is the sick snake. If you find runny poop, which snake are you gonna bring in to the vet? Are you gonna treat them both to stressful medications?
No need to be protective Nomes, as we're only here to guide. I'm sure all of us don't wanna see another herper go through the same mistakes we've made. Take everything here with a grain of salt, cos remember, the most important thing is the health of your snakes not your or my opinion.