Chapter Tactics when using two Space Marine special characters?

Since the codex doesn't specify the timing of choosing which to use I would say it would be up to the TO.

Personally I would say you must stick with one for the duration of the tourney.

The question that needs to be asked is whether it gains the player an unfair advantage to tailor his army for each opponent before each game. Not sure how it can be answered with a "no".

But by doing so fairly, which means you stick with the same army from game to game.

It's the same as changing psychic powers from game to game, tailoring them to your opponent.

You're making a choice between having your entire army Fleet, Outflank, or Stubborn. Which are of different uses for different enemies, thus tailoring your list between games.

@Dave- I was specifically talking about Librarian powers, which you just pick from a list and do not pay points for.

As for deciding about Outflank and Deepstrike and such, none of these abilities are lost by making the decision to use them or not, Chapter Tactics are a "this or that" decision, which is changing your army list IMHO.

Bottom line? Ask the TO beforehand.