Introduction:
There's three distinct ways that one can build a team in TTT2. 1) Pick a secondary character you're most familiar with or a character that you may not be familiar with but really like 2) Pick a secondary character that complements your character 3) Pick a character that is the complete opposite of your character.
Option 1:
This is the best option for beginner players and experienced players who don't necessarily care about the new netsu system, but would rather play whoever they want to play. If you are a newer player, you should definitely pick your secondary character to be whoever you are most familiar with. For example, if you just started playing and picked up Paul and a few weeks later you decided Lili is more your cup of tea, you should probably pick Paul to be your secondary character until you have a better grasp of the game. Relationship wise Paul and Lili have nothing in common and as such probably don't have any non-generic tag attacks and what not, but you are more likely to win with Lili than picking up Law to complement Paul, with their extra tag throws and attacks. Play whoever you are most comfortable with.
Option 2:
Picking a secondary character that complements your character doesn't necessarily mean picking a character that covers the major deficiencies of your point character, which is option 3. Rather it refers to picking a character that can greatly assist your character during TA or a character who "plays" similarly to your point character, but isn't nearly identical in strengths and weaknesses.
A great example of the first case is the character Jack. While he is fairly simple to play and his only glaring difference from much of the cast is his size (big) and reach (pretty far), he has one of the best TA "assists" in the game. Jack's u+1+2 knocks the opponent high into the air allowing characters with U/F~4 attacks to use one of the strongest filler attacks in the game to extend their combo, an example can be found here. While I do believe most characters will have a signature/BnB TA that they will be defined by, I do believe that that will not all be equal in universal usefulness.
The second case is characters who complement each other without huge differences in move list and execution. A few examples would be Asuka and Jun, the Mishimas as a whole, Baek and Hwoarang, etc. While their fighting styles (as groups) are similar, they each have unique strengths/weaknesses. One such example would be Heihachi and Kazuya, Heihachi has good mids and poor lows, while Kazuya has better lows and poor mids. The major benefit in TTT2 is that these teams will also have unique team attacks and throws that can't be activated normally. These attacks often destroy red health making defensive tagging less useful as their hurt character will be unable to recover as much health.
In TTT, teams with some type of relationships would activate netsu earlier (or possibly not at all) making such teams naturally strong(er) versus a randomly thrown together team (e.g. option 2 versus option 1). Supposedly that has changed in TTT2 as some characters will naturally activate netsu earlier versus characters who will activate netsu based more similarly on T6/BR rage rules. If that's the case, option 1 and option 2 will have (near) equal weight of success, making a more balanced game. The only glaring weakness to option 2, though, is that similar characters will make player patterns easier to pick up on in the long run.
Option 3:
Picking a secondary character that is very different than your main character is a strong strategy when you can identify deficiencies in your character and your overall play style. For instance, if you play Ling Xiayou, a deficiency in that character is her reach making Jack a good complement in both regards to option 3 and option 2 (to use his TA to extend her carry range). If you play Dragunov, a deficiency in that character is a complete lack of NC KND/W! attacks, you can pick Jinpachi who possibly has the most NC KND/W! in the game. If you play Paul, whose deficiency is that he's a fairly straight forward character, you can play Zafina/Raven/Lei who have numerous tricks and stances to confuse your opponent.
As I mentioned at the end of option 2, playing similar characters can cause you to fall into patterns making your game easier to predict. That isn't necessarily the case for option 3 as completely opposite characters tend to have completely different play styles. The only glaring weakness of playing very different characters is that it requires you to understand how to punish two, possibly very different, ways. You might have to know that character 1 can punish with f+2,4 while character 2 can only jab punish, or maybe character 1 can punish with b+4,3 while character 2 can't punish that at all. Worse case scenario you confuse yourself and throw out a punish that would normally work with character 1, but is some horrendous attack, especially on whiff while it leaves you right in the face of your opponent. It's happened to the best of us and will continue to happen long into TTT2's life.