Flare Blitz in 2 VS. 2 is only really decent if Team Attack is off. Let's say you are on the left side of the stage, and the other three players are on the right side of the stage. Since Flare Blitz stops as soon as it hits a target, not only do you have to make sure that the closest character to you is someone other than your teammate in order for Flare Blitz to actually do anything good, but this move is only really rewarding if the opponent of yours who you hit happens to be at a high enough % to be KO'd by the Flare Blitz, and this is extremely difficult to ensure given how chaotic 2 VS. 2 battles are.
To make this clearer, say you are on the left side of the stage, and to the right side of the stage are your teammate, an opponent who has low %, and an opponent who has high %. Unless two or more of the aforementioned characters are very close to each other, Flare Blitz can only hit one of them: the one closest to Charizard. In the aforementioned situation, the character you'd want to hit is the opponent who has high %, but as anyone can roll and move around, it is very difficult to ensure that another character does not get between you and the character you want to hit.
There are two primary reasons why I think Charizard is not as good in 2 VS. 2 (and FFA, for that matter) as in 1 VS. 1:
1. Charizard relies heavily on Flamethrower, pummels and throws in order to rack up damage in 1 VS. 1. When Charizard uses the aforementioned moves in 1 VS. 1, Charizard's single opponent is more or less completely helpless, and since there is no one else to attack Charizard while it is using Flamethrower, pummels or throws, the fact that Charizard leaves itself completely open to being attacked by a third character while it is busy trapping and attacking one character is completely irrelevant in 1 VS. 1. However, the fact that Charizard leaves itself open in such a way so often is extremely exploitable in multiplayer matches, allowing it to be easily hit by even very slow attacks such as Falcon Punch and Ike's Fsmash.
2. One of the things that makes Charizard such an amazing character in 1 VS. 1 is its near-unparalleled edgeguarding game. A significant amount of the damage that Charizard typically deals in 1 VS. 1 comes from throwing the opponent off-stage, damaging them with Flamethrower while they are trying to recover, reading the way they get up from the ledge in order to launch them back off-stage (for example, hitting them with Fair out of their ledgejump, shieldgrabbing their ledge getup attack and throwing them back off, or throwing a Nair above the ledge to hit them back off the stage as they try to climb up), and things like that. But of course, all of this becomes completely irrelevant when there is another opponent on the stage ready to attack Charizard while it is edgeguarding.
The best characters in multi-player matches are those who don't need to leave themselves open to being attacked by a third character very often in order to deal significant damage to one character, which is why I think Captain Falcon and Ganondorf are among the best characters in 2 VS. 2, especially given how easily they can throw extremely powerful KO moves at the opponents while they are busy fighting Captain Falcon's/Ganondorf's teammate.
With that said though, Charizard + Falco seems to make a quite potent team, based on the testings of myself and another Smasher known as Kal1978. I don't know exactly why that is the case though.
