"If you connect them together, they are DEAD SHORTED. Whether anything

"happens" doesn't change that fact. "

But the fact is the OP had a problem where he gets an arc and trip when
he closes a disconnect. That indicates a short causing high current.
That can't happen because two hots on the same leg happen to be
connected together, resulting in zero current flow. It has to be a
short from the hot to either neutral or ground.