Target mage 1 and type /focus, this puts his Unit Frame on your screen, then just target the other mage all the time, you now have 3 Unit Frames on screen, the Shamans, Mage 1 and Mage 2. Make a macro to target Mage 1, and another to /focus on Mage 2 (so you can set it up fast when you log in)

Have 2 bars using Trinity Bars adon. You can specify a target to a bar, so anything you click on that bar will always cast on that target so:

Bar 1: 6 buttons, all auto-directed at Mage 1, Keybinds 1-6
Bar 2: 6 buttons, all auto-directed at Mage 2, Keybinds 7-=

(you can have as many bars and buttons as you like per target)

Job done, only 1 macro at start up that casts /focus then /target Mage 2. And an adon that does the targetting by bar for you