I have a relatively good time on my dual-boxed shammies!
They are currently 73.5 and just finished the two starting areas in WotLK, getting ready to go start the dragonblight map.
One up front is spec'd as Enhancement, and the one in back is an elemental. The elemental's main spam macro has two /castrandoms with extra commas in them. First /castrandom is for DPS, the second /castrandom "sometimes" throws a heal to the front guy.
When I occassionally run with my Mage buddy, I do notice that he out DPS's me by quite a bit. The enhancement shammy seems to out dps the elemental, but last level or so, it has been very close...
If you like shammies, I don't see a reason not to play a pair !!!