sounds like they would be cutting the original tip off and making a wider opening for the dual tips and welding them on. i'd ask them to see the cost because it might make more sense for them to make you a quad tip exhaust. it's what I have. I had them do a catback, resonator delete and muffler delete nd i'm happy with it.
only thing i'd be worried about is if the roush exhaust would still sound good after modifying it
2014 Ruby Red GT A6 - 3" Magnaflow Quad tip Catback