Hey all, been reading the forums here for years, but this is my first post.
Anyway, I have almost the same exact situation. Main is a WAR, and made a shammy to two box. I went with a Mag for a leveling partner and have to agree that that was a great choice. The Mags pets and nukes take down mobs fast, and are nearly unstopable with shammy buffs. I got to lvl 30 with them in a about a week, and I am only a casual player. Anyway, the Mag is still stuck in the 50's some where because the SHM started grouping with the WAR as soon as he could get exp. I still play the MAG when I am burned out and feel like soloing. Also, I was camping some items in Bloodfields, and tried the MAG/SHM after I used the SHM/WAR to break the rooms, and the 2 pets would easily tank/kill the trash and named before repops. That was back in the Luitienent (spelling) room. They are a fun class to play together IMO.
*Just a side note, and kinda a funny story, I made the MAG and of course did not guild him, and ran into another MAG one day and we instantly became friends. We leveled together for about a month beofre we started talking about mains. We'll turns out that not only do we know each other from our mains, but we are also good friends, and officers together in the same guild

. Just goes to show, its not the toons that you are playing with, but the peeps behind the glass.