are you using Form or ActiveX combo boxes?
The short answer to your question is to create a hidden vlookup table off to the side, or on another sheet. Two columns, the first is the concatenated results of all the possible combobox combinations, and the second column is your desired xxxxx value. Say you were picking out articles of clothing in your two boxes, your first column might contain entries like: ShoeHat, GloveShirt, ShirtGlove, etc
The two combobox types output their results to excel differently. A Form box has a linked cell property if you right click it, but it will only output the index number of the chosen item, you would then have to look this up against your list to see what the item number is. The Activex combobox has a linked cell property, and everytime the combo value changes it will write the same value to your desired worksheet cell. From there, it's a simple matter to concatentate the two linked cells together, then do the vlookup.