I would be rather surprised if it wasn't possible, even with different hemipenis design. When you think of the crazy hybrids that some people have decided to create, like bloodballs, you can imagine it would be rather straight forward with garters. But then again garters else a completely different species and boids are classed as primitive snakes so I don't know if they abide by different rules when it comes to hybrids.

Something to think about for the day though. Have many garter hybrids been found naturally in tin wild?