I have this doubt: if I wield two Fortuitous weapons and make a trip, then hit the AoO I can make two more AoO at -5 (one with each weapon)?
Just to get it out of the way, Greater Trip doesn’t really change anything here, it just provides a consistent way of getting enemies to provoke attacks of opportunity from you.
The question then becomes, “if I wield multiple fortuitous weapons, can I make that many extra attacks of opportunity from a single provocation?” Unfortunately, the answer to this question is unclear.
Once per round, when the wielder of a fortuitous weapon hits with an attack of opportunity, he can make a second attack of opportunity with this weapon against that foe at a –5 penalty.
The “Once per round” restriction is clear enough in the case of a single fortuitous weapon. When you have multiple, however, you have multiple abilties that each say “Once per round,” and it isn’t clear whether they should be independent (each getting that ability once per round) or shared (with the once per round limitation applying to the character). To be sure, “Once per round, [...] he” certainly applies the limitation on the character, not the weapon—but if there are two weapons saying this, should it get doubled or not?
The rules just aren’t clear enough to say with authority. You will, ultimately, have to ask your GM.
However, considering that you have to pay for fortuitous twice in order to get that benefit, it seems entirely reasonable that you would get, well, twice the benefit. And, for that matter, consider also that applying it to two weapons implies using two weapons—that is, not using a big two-hander. That means each attack is weaker than the two-hander’s single attack would be, and also either means you have no reach (a huge drawback for a character who cares about attacks of opportunity), or you’ve spent a feat or awkward class level getting proficiency in an exotic reach weapon you can use with one hand (i.e. you have paid quite a lot more than just double fortuitous’s cost here). Considering the costs and/or drawbacks involved here, I would personally say Yes, you can effectively get three attacks of opportunity in a row this way.