Their intent seems to be 2 tanks for each boss, but at the gear level we had when testing the only way to make enrages on the last two was just to use one tank and zerg them.
The third boss has a target switch mechanic that should be handled with two, and I assume their intent was to have the second tank working on the puzzle.
Kephess has a few tank swap mechanics as well, but you can't make one of the dps reqs of the encounter with full rakata gear. So we just hit him over and over with one tank until the tank lived long enough to get him down, the live release might have higher damage output and make this impossible.
So yes right now in full rakata you can't kill the last two bosses with two tanks, but as people move into the next tier of gear two will work best.
Note:
Yes i'm credible, see twitch.tv/mywiddlepony for our videos, or ambient.enjin.com, we have been ptr testing.