I think GSDs can have rear dews. NEVER heard of Sibes having them, though. Not purebred anyway. There are many people crossing Sibes and GSDs for one of those superbreeds, though.
I've seen a rottie and GSD mix with rear dews. He just looks like a big red dog, like Jazzmonster. Would never guess the cross.
My roommate had a GSD and Sibe cross with double dews in front, and back dews too. she looked like she might have a bit of collie and/or rottie in her too.
I've seen lots of mixes with them, actually. But then again, I work on the shelter scene.