I have recently obtained an interest in creating a tier list specifically for Tag teams. This list would be much longer than the single-CO list simply by necessity. After all, there are 27 COs each with 26 potential partners. That's 702 Tag duos that I'm aiming to rank on a single list. (No one can ever accuse me of thinking small again. XD) I suppose if I really wanted to be a completist, I could throw the single COs onto that list as well. After all, some of them could potentially hold off some of the more ineffectual duos.
In the single CO tier list, COs for the most part are put into little groups that are then ranked against the other groups. COs are then ranked within their own group, and COs who don't fit in a particular group are then ranked against the groups and each other. These rankings are mostly based on the effects of Supers on the battlefield, with only a few notable exceptions. The Recon COs, for example, are so named for all of their Supers significantly amping up the damage Recons can do in the early game. Similarly, mass damage COs are so named for their Supers dealing free damage to large numbers of units.
In a Tag list, however, we must think in slightly different terms. The first and foremost thing that must be drilled into one's head when talking about Tags is that it essentially gives every CO in a Tag duo the Super CO Power "Lightning Strike", except that footsoldiers also get a second move. With few exceptions these Tags come earlier than Lightning Strike, and all of them have additional effects piled on top. As such, even the worst Tag duo is better than single-CO Eagle by default.
The second thing that must be considered in a Tag list is that you cannot look at power alone. The rate at which a Tag charges can be just as important. On the forum, the question came up about whether Andy or Hawke would be a better partner for Rachel. While it is inarguably certain that Rachel/Hawke is better pound for pound, during an actual game Andy/Rachel will charge up as early as two days sooner. Since Rachel deals free damage in such massive amounts, the battles occurring during this Dual Strike are not necessarily guaranteed to grant Rachel/Hawke their own Dual Strike, especially if Hawke is caught on the back burner charging at 3/4 the normal rate. Simply by virtue of coming sooner, Andy/Rachel can cause enough damage that the retaliatory Dual Strike, despite being stronger on paper, simply won't do as much since many front-line combat units have been destroyed.
The following are some early thoughts of mine on how this might begin to shape up. Bear in mind this is just sort of "calling it now" and could easily turn out to have no relevance at all:
-First of all, I think that mass damage is going to find itself interspersed with mass spam and super units as far as placing Tag duos at the top. I predict this due to the fact that unlike in single-CO play, you have two turns to capitalize on the free damage. Additionally, mass damage duos have the added benefit of dealing twice the pain and thrice the damage since their Supers accumulate. Drake/Olaf is the often cited example, but even Hachi/Sensei is going to find itself hard pressed against Olaf/Rachel.
-On the topic of spam, I suspect that in certain situations its effectiveness is going to decrease. Specifically, spam vs the ability to tech up quickly. While Sensei/Hachi can undoubtedly make a large, teched-up army, they actually limit each other's capabilities pretty severely. Sensei must go first, and he cannot buy anything. As a result, none of Hachi's teched up units get to move that turn. In single-CO play this isn't a big deal for Hachi, but in dual-CO it makes this team vulnerable to a retaliatory Tag. As previously mentioned, mass-damage can utterly crush them during this vulnerable phase, especially Rachel's missiles since a lucky shot can turn a Megatank from a harbinger of doom to a juicy target. When Uber Lightning Strikes are a regular occurrence, even getting Black Bombs and bigger guns on the cheap might do nothing more than simply even the odds.
-HOWEVER, while the pure spam Grizzled Vets may not be as effective in the face of such heavy hitters, using spam as an enhancement to other kinds of Supers still makes them dangerous in dual-CO play. You don't know true horror until you've seen Sami/Sensei and Hachi/Kanbei in action.
-As mentioned earlier, speed is important. I heavily suspect that Adder/Koal will have a relatively high place in the tier list simply due to being the fastest-charging Tag in the game. (Well, granted, you could substitute in Sonja for either of them and still charge at the same speed, but you get my meaning.) The ability to dismantle a large chunk of the opposing force before they're able to exercise their own firepower on you is a critical component of dual-CO play which you ignore at your own peril. With only a few notable exceptions, speed kills in this mode.
I'll start making this list on the forum inch by inch starting with more obvious Tags, and then working my way into more obscure ones.