Quote Originally Posted by Ualaa
I wouldn't have allowed that at my table.

Feint Strike allows an attack as a Swift action -OR- as an attack of opportunity (if you expend focus). It never says you can do both a Swift action and then expend your focus for another attack as an attack of opportunity.

The rest of the combo stands, and being able to close the distance on a target, achieve a feint, and deal two attacks, possibly retaining your Swift action in the process...

That's a strong combination, as it sits.
Sorry I didn't explain what the synergy was properly.

The Drow had Fencing Sphere, Fast Feint, Feint Strike, Focusing Feint, Verbal Feint, Dual Wielding Sphere, Dual Opportunity, Mercurial Flow, and Great Focus.

First he used Fast Feint (move action, expend 1st focus) to feint a PC and move adjacent. Then he used Feint Strike (AoO, expend 2nd focus) to get an AoO and Dual Opportunity to also attack with off-hand weapon. Next Focusing Feint (Swift Action) regained the 2nd focus. Finally the Attack Action allowed both main-hand and off-hand attacks for a total of 4 attacks. With Sneak Attack and Sphere Specialization [Dual Wielding] this became 4 attacks at full bab with a fair amount of extra d6. My main point is that there is plenty of synergy between the extra attacks granted by Dual Wielding, the Fencing Sphere's ability to generate opportunity attacks, and any source of extra damage you can leverage.