It can't be done with a Combo Box.
Options are
1) Use a list box
2) This is a bit tricky, but you have 2 text boxes, the Control source of the first one is
=MyCombo.Column(0) and the second text box is =MyCombo.Column(1)
Where MyCombo is the name of your Combo Box
Now the trick is to make the width of the first text box the same as the width of the combo box less the little grey dropdown bit on the right of the combo box. Lay this text box exactly over the Combo Box so that just the grey bit shows. Use the arrange to bring the text box to the front. Set the text box to locked.
Here is a picture where I haven't overlapped them completely.
Note that the fonts must all be the same.
Phil