I am joined into a friends game. When I use the crafting bench he cannot use it. Howerver if he is using it I can then open the crafting bench and start using it. However if he sends a crafting request it goes through and instead uses my materials to craft. It does not use his and he does not receive any materials. Mine are however used and I see them.
Steps to reproduce.
1. Join multiplayer.
2. Host uses crafting bench.
3. Another player opens the crafting bench.
4. Have the host send a craft request such as make iron ingots.
5. The result is your ore is used to make ingots in your inventory, and not his.
The reverse is not the same, the host cannot use the bench if the other player is using it. As well as the non-host when trying to craft iron ingots does not cause items to be crafted for the host. This does work with any crafting as far as I can see