i have three wild caught garters and i am wondering if i could keep two of them in the same cage it is large in comparison to them and they are near to the same size.

The largest garter is feeding fine it has eaten two slugs.
but the two smaller ones have not yet. i am wondering what size of a slug or earthworm they can eat they are both about 15 inches long. i have heard that garters are sometimes cannibalistic during feeding. do i run the risk of them eating each other if they are kept together.