My '88 C3500 SLE with a 454 had dual tips, was factory. I thought either the BB or 350 trucks got them. Didn't see that as an option within the literature I bought for it but that doesn't mean it wasn't optional..
My '89 K1500 Scottsdale with a 350 has dual tips, but it's not the factory exhaust. Looks factory though, like my '88 did.
Like others said, fake duals. Both trucks have a Y pipe that necks down into what looks to be a 3" pipe or around there. Hits the muffler near the axle, then splits into the fake duals.