There's three ways to tank phase 1:
1) solo tank it. Have the OT help pick stuff up and interrupt the shaman but otherwise just let the MT tank it all. This will require some really good cooldown usage by the MT, especially during warsong.
2) 1 tank on boss, the other on adds... as much as possible. Similar to strat one, but the OT will constantly taunt off all adds as much as possible when warsong hits and just fight for as much aggro as possible. If you have a prot war they can use vigilance to infinite taunt adds off during a warsong (usually the 2nd).
3) Tank Swap: What this is is one tank starts off on the boss, builds vengeance, when the adds spawn, both tanks help pick up adds (which is really best for any scenario) but then then MT will AOE threat to get the adds, then when garrosh does his warsong to empower the adds, the OT (who before this should have no aggro on anything) taunts garrosh, and then its his/her turn to build up vengeance.
The 3rd is probably the most elegant, my guild used #2, mainly because I wasn't raid leading and tanks get threat epeen/damage happy sometimes and wanna see that 500k+ dps and don't care if the OT is floundering around 100k dps b/c he/she doesn't have threat on anything. I've done #1 before too in PuGs.