A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
I'd worry that DL will notice or find the double booking. They could cancel both tickets.
You won't be able to keep the seat next to you as that would require that both boarding passes be scanned at the gate. I don't know whether DL would let you change the first name to "ExtraSeat" to make this possible. It might require that both tickets be on the same PNR.