Example for 1:

I got two canteens, both of them empty and I want to fill them up.

1. Get one canteen in character hands.
2. Fill it up.
3. Drop the full canteen to the ground.
4. Get the second canteen.
5. Fill it up.
6. Put second canteen away in the inventary.
7. Catch the first canteen from the ground.

Same would be to drink or emptying them.

Example for 2:

I've got two canteens in my inventory and I want to put them in numeric menu, one is going to be on number 8 and the second one in number 9. I have one canteen on my jacket and the second one in my backpack. The one in my jacket is put in number 8 while the one in the backpack is put in number 9. When I click on number 8 or 9 both numbers appear as selected and in the inventory it is selected just one of both canteens. It does not matter if I click on number 8 or 9, it always appears the same canteen as selected.