Picked up foreign salve will stack with dug one in inventory, and share both properties - inability to use, as you don't own enemy's salve, and invulnerability for denying, as one salve been dig out from the shovel. And all salves that you will dig and pick up then will stack with this one and be disabled too, until you put out stacked salves from your inventory.

1. Dig a salve with trusty shovel and pick it up
2. Pick up an enemy's thrown salve from ground
3. Now your salve stacked with enemy's one

Both salves wouldn't stack at all, or all salve stack would be usable.