Really depends what you want to do. Mages can already do some impressive solo ZF/Mara/BRD runs. Hunters solo tribute & Princess runs. Mages/LocksH Priests already rule DM:E solo. Adding a 2nd doesn't really make any of that easier/better, as a lot of those runs rely on some tricky jumps that may not be easily dual-boxable.

I do think 2 Mages will be great when ZG comes out - good for croc farming. But beyond places where 1 mage is almost not enough and a 2nd mage for more raw dps will help - I don't really think 2 mages will help you sell runs better than a single player. Having some sort of healer (even hybrid) or lock for summons may help you sell more runs.