At the NHL level, most coaches still play matchups though. You pit your defenseman best suited for the assignment against the other team's top line, especially at home. And in Detroit, even with Petry, that is going to be Seider. He's going to get the other team's top line. It's of course great to have backup for that role, especially on the road, and I guess Petry could be the first guy to jump on the ice when it can't be Seider. But that's all he would be in general, the first alternate for the toughest defensive assignments.
If the Wings are losing? Sure, I can see skewing Seider towards O-zone starts and Petry towards D-zone starts, in order to make something happen offensively, but we're getting awfully particular here. In general, Petry shouldn't be eating into Seider's defensive assignments. Seider is easily the better defender.
It's entirely possible to have your #1 and #2 dmen both be RHD, or LHD, it does happen. But given all the context around the possibility of a Petry acquisition, and what it would mean for Detroit's D corps, this is pretty square peg in a round hole. And this all feels weird, because we are comparing apples to oranges. Petry is not, and has never been, on the level of Moe Seider. They are in different tiers, different classes. Petry may easily be the 2nd best defenseman on Detroit, but he's still not close to Seider, even defensively.