Hmmm... now we're talking! And now we are getting in to some interesting wording as well, or at least in the swedish version. Could some one tell me what the english version says for ambidextrious level on; does it say "one attack per round" or "an attack per round"?Well, Swordfighter Tier 2 + Ambidextrous Tier 2 would actually allow the PC to make four attacks. Swordfighter Tier 2 states that the character can SLASH two opponents with the same action (rolled for as separate attacks) but says nothing about what kind of action. So the extra fast action attack from Ambidextrous Tier 2 would allow the character to make two additional SLASHes with a -2 modification. Ambidextrous Tier 3 removes the modification, so four SLASH attacks in one round. Makes three stabs with a knife seem quite reasonable, don't you think?This should be the correct interpretation.The RAW is no, because you only have a Max of two fast actions per turn.
Knife fighter is for if you want more accuracy but lower base damage. Two attacks per round with d8 artifact die but only 1 damage.
Ambidextrous is for if you want to attack twice with more powerful weapons.
Ambidextrious, at least how it's worded in the swedish version, does not let you attack twice on one action. It let's you use your fast action to perform the second attack.
So since both ambidextrious and knife fighter only let's you use short actions for attacks, you are pretty much limited to only 2 attacks per round with this combination.
To compare, Sword fighter lets you do two attacks (Against different enemies) as part of the same action. So if you had Sword fighter and ambidextrous, you could first attack two different enemies with your sword, and then perform a third attack (As your second action) against either one or even a third target.
While I agree with your interpretation, I think some GMs might take issue with the swedish version saying "one attack".
This does make me inclined to agree knife fighter + ambidextrious should grant three attacks together despite the wording there being more clear against it.