AND...
They will play against each other the second pre-season game next sunday.
Is there an antecedent to this?

I have never seen in any sport that two teams practice together and much less prior to a game they will match.
And even more incredible is that there are talks that the COWBOYS and Chargers will make this an ANNUAL event

I know there are links between the team because of Norv Turner, Chargers' headcoach, cuz he coached in the COWBOYS and was a mentor to Jason Garrett and Rob Ryan, but it seems a little weird.