I think I understand your question but it is vaguely worded so this is a guess as to what you wanted. If your combo box is based on a query or table, you can return more than one value to the combo box at the same time (all from the same row, but different fields.)
So you build your combo box using the control wizard. It will ask you where to get the data. You tell it. It will look at the table/query and give you a list of values it could return for you. You identify what you want. Let's say for the sake of argument, you have two values. One for the combo box and one for the text box. Let's further say that this combo-box is set for single-select, not multi-select. That is, you can only select one row at a time.
So now you select the row. Inside the combo box, you have TWO columns (because we said that you were going to ask for two values from the same record.) Columns are zero-based for Combo Boxes. The selected item is visible in the .ListIndex property of the combo box.
Normally, the item that you selected as the first field to be returned is also the one that is visible in the combo box. If the combo box name is CBX, then the item that is visible after you selected something is
CBX.Column(0,CBX.ListIndex)
In the control's Click event or its LostFocus even, you could load the text box (call it TBX) by doing this:
TBX = CBX.Column(1,CBX.ListIndex)
This is a glossy overview. Play with it a bit. Read up on the properties of Combo Boxes before you play with them too much. But this is the main part of what you asked, I think.