Answered: Alliance selection

According to some people, two or more teams that have already qualified to the World Championship may not be in the same alliance during the tournament playoffs.

Example:
Team A and B qualified to Dallas
After the qualifying rounds of a tournament, team A picks team B during alliance selections.

Is this allowed or not? (Note that according to the rules in alliance selections, it does not mention anything about this.)

Thanks

There is NO rule which states this. This is a legal move.
The only rules about alliance selection are the ones listed in the Tournament section of the manual.