While I think maximizing single player xp/hr is interesting, I'm more interested in the best places to earn xp in total considered across all characters participating.

For instance, Sam is earning 22 million xp/hr since there are two toons in the tag group. Assuming that he's going to eventually want both of them to 90, all of the experience is meaningful progress towards that goal. Also Sam, you'd get more total xp/hr, but less per alt if you had a full group of 5 tagging due to group xp multipliers. (If these are your last two to level, nevermind that suggestion)