Looks like two casings pushed together. I found two like that last year and was going to pull them apart to see what sorta treasure might be inside but never got around to doing it.
I was thinking two cases pushed together as well.I two of the all brass shotgun shells pushed together and they had gold inside.My cousin found two rifle cartridges stuck together and they had a rolled up paper hunting Lisc. from 1913!!
We used to put a fire cracker inside with the fuse sticking out where the firing pin was knocked out and push another shell into it. Mount the 2 shells in a vice grip pliers and mount the pliers to something solid.. Aim it, light the fuse and fire! We used to have fun shooting at an abandoned vehicle in the back yard. Worked well as I recall. I dont believe the things we did as children.