Just in case there isn't a way (I would love to know if there was a way) I'll tell you how I would do it. Change the font of the combo box to Courier (this way all spacing is the same) and then combine your entries before you add them; then use the itemdata to store the unique holder that you will refer to with your ListIndex property.

dim string1 as string * 20
dim string2 as string * 20

string 1 = "First Column"
string 2 = "Second Column"
cboCombo.additem string1 & string2
cboCombo.itemdata(cboCombo.newIndex) = You're unique numeric identifier