Passing values from databound DropDownList to databound TextBox

Let's say I have DropDownList ddlRooms bound to the datasource which returns list of available rooms from table tblRooms.
On the other hand I have users, and their info is stored in table tblUsers. Using FormView frmUsers can be inserted, edited and updated... frmUsers is be bound to separate datasource...
To each user, I would ilke to assign a room, so the value of txtUserRoom will be populated based on the selected value from ddlRooms.
How can i pass the selected value of ddlRooms to txtUserRoom while keeping txtUserRoom stil bound to user datasource? Do I need two separate fileds or can i just put value of ddlRooms into datasource user?
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