1. Put Item in a container (car trunk, crate, etc)
2. Both Player 1 and Player 2 should left click and drag Item over their backpack
3. Both Players should release left mouse button simultaneously to drop Item into backpacks
4. Now Item duplicated - both players have the Item in their backpacks.

After this manipulation it seems item have the same ID until one of the players re-enter the server. I.e. if you clone backpack and start to put items in it - items will appear in both backpacks.