Any duo is going to do better than solo. Shaman + Mage basically means you have your Shaman Slow Tanking while the Shaman + Mage Pets beat on the mob from behind. The Mage will nuke via mana and/or clickies.
Mages have no real CC, so extra DPS is about all you are going to get from this combo, as people have already mentioned above. Shamans can already summon food/drink (albeit slowly), and bandages can be summoned through
https://wiki.project1999.com/Embalmers_Skinning_Knife if the Shaman wants to bind wound. Mod Rods have a bit better ratio than cannibalize, but realistically the Mage is going to need the mana themselves for nuking, and summoning a bunch of Mod Rods takes time. The Shaman is getting no benefit from the Mod Rods. Muzzle of Mardu isn't going to give the Shaman pet a huge boost in DPS either.
This duo is like a Shaman/Rogue duo, but without the benefits of lockpick + sneak.