It’s not completely rare, however, as The Flash and Supergirl crossed over a few years back and Ally McBeal and The Practice did long before that.
The winner in this instance would get to have its story stretch between two shows on two unrelated networks in a television event that doesn’t happen all that often.
The reason Dick Wolf hasn’t exactly decided which shows will be crossing over between networks is due to the fact that he hasn’t figured out “which story wins.”