A SqD 3 phase breaker will fit in a single phase panel.
(at least the old ones, there may be some kind of rejection device now)
I suppose it would allow your 5-15/6-15 device to be on separate circuits and still be compliant but I bet it is a 110.3(B) violation.
Usually this arrangement is when you have a 120v outlet on a 240v single phase circuit where a neutral is provided.
This is a multiwire circuit with a 240v load if you want to look at it the other way.
It is not really unusual at all. Think about the light in your oven.
