I just took your name in vain in the other post about Shivers.
As for Watchmaker -
Open inventory using the TAB key.
Left click on the name of the character at top of screen. This will cause the Inventory list od the second character to appear. The left click on an item name on the list will cause that item to leave the second character's list and move to the initial player's inventory. - No, they do not be in the same room.
Other stuff that confused me that you might want to know -----
If the characters are in different room - above the inventory item list, a button with a telephone will appear. Click on this button will cause the other character to come to that location. That telephone won't appear if the other character is in the same room already.
To change from one character to another - press F8 key.
To make them run - shift key.
To change visuals space bar to zoom.
To lean forward - arrow or cursor with shift key (very important in one place in the game).
To crouch - Z key.
To raise the view - A key.
More than you ever want to know.