You may be better off using multiple dominoes instead. You aren't going to gain much by making a thicker tenon because you are still limited by the cutter depth. Using multiple dominos and referencing from opposite sides would certainly by a lot easier. Even if you were able to stack the mortises you would still have to clean out the sides (ends of the cuts). I assume that you are using a DF500 and wish you had a DF700 XL? I have seen people do multiple mortise cuts side to side then use a traditional tenon.