A family of Microsoft relational database management systems designed for ease of use.
A Combobox will display ONLY the first non zero width column after selection. So you have a few choices. First, the ID is usually system generated and the user doesn't need to see it. If that is the case, then set the Column Widths property to 0";2" (or whatever width you need for the username). The user will only see the Username, but the ID will be stored.
If that doesn't suit, then you could concatenate the two in your RowSource as Hans suggested, again setting the first column to zero width.
Another solution is to have a text box with the ControlSource of:
=combo.Column(1)
This will display what is in the second column (count starts with 0) in the text box.