I think most people here will agree that monk/shaman will end up being the more powerful duo in the end of the two you are considering. The issue of not being able to gate is irrelavent once you both get WC gate caps and the shaman can just help the monk recharge every use. Monk shaman can also level with almost no gear investment but weapons and a haste item for the monk will speed things up tremendously.
Another thing to consider is your end game goals. Although monk and shaman are extremely powerful solo and duo for that exact reason the server is flooded with them , especially bad ones. At this point in the game I encounter far fewer mages and necros than monks and shamans. You practically cant go anywhere without seeing monks and shamans. Anything you play will be viable on raids though you may have an easier time finding a raiding guild as necro/mage than you will as monk/shaman.
As someone stated earlier cleric/enchanter is also something to consider although I see you arent interested in other combos. Not only would this combo likely have the easiest time finding a raid guild together, you also have two of the three classes of the EQ trinity (cleric, enchanter, warrior) for forming groups and argueably be the strongest duo of the three listed depending on what targets you plan on hitting.
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