It's been a few years years since I've taken them, but if I remember correctly there would be no overlap at all. Backstage Safari takes place solely at Animal Kingdom. Backstage Magic (unless it's changed) takes you only to Epcot, Disney Studios, and Magic Kingdom. It skips Animal Kingdom.
Backstage Safari takes you backstage at Animal Kingdom where you see the animal barns, we got to pet a white Rhino named Harley, you see some of the food they grow for the animals, visit the area where they prepare the food for the animals, vist backstage at Conservation Station and get to talk to a vet, we got to pet a spiny lizard there, they then took us on our private safari, and we got a snack. It was a good tour, but that's really all I remember about it.
Backstage Magic, we got on a big bus and they took us backstage at Epcot, into the German pavillion, then around to the American pavillion, where we went beneath the American Adventure to see the animatronics they use in the show, then back up to the outside of the American Adventure building, then back backstage to a CM building, where they have all the uniforms the CMs can sign out, then we got back on the bus and went over to DS where we got to go into the bottom area of Tower of Terror to see how it works, then we ate lunch at Whispering Canyon (think lunch is at DS now though), then over to MK where we saw some of the utilidors, then went into another CM area, then another building where they sort laundry, then another building where they sew costumes, then another building where they refurbish ride vehicles, carousel horses, animatronics, etc, and then we were taken out to see the afternoon parade. We probably saw some more stuff I'm forgetting, but do know we didn't go anywhere near AK that day.
Oh one thing, on another trip I did the KTTK tour and we actually went down into the Utilidors and walked around, but on Backstage Magic, we just took a quick peak down one of the tunnels. We didn't actually go down into the Utilidors.