Giving twin strike to a fighter

[Zurai]

Another objection to it is that a Defender Fighter could use his shield as an improvised weapon to take advantage of it without giving up his weapon-and-shield perks.

No, he can't. Shields are too heavy to be used as one-handed improvised weapons.

 

[Tony Vargas]

Indeed, the unarmed version would have its "[W]" limited to 1d8/1d8 probably, and could be limited to 1d8/1d6 I suppose. ... That should definitely help keep it from getting out of control, and as you say, it could always be adjusted later if in play it turns out too good.

Another thing that occurs to me is that he's supposed to have the defender role. His AoOs won't be as damaging as those of a greatweapon fighter, and his AC won't be as good as a gaurdian fighter - but, his ability to make two attacks with Flurry of Blows, would let him mark more enemies, which might make up for it.

You could also give him a more Monkish feel by replacing the Fighter feature that stops enemies moves with a successful OA, with one that renders enemies prone with a successful OA, but only inflicts STR bonus damage, no damage roll (so no crits, or other bonuses). Prone is stronger than movement cancellation, but damage is lower - and, it's a martial arts 'throw.' Again, if it proves too powerful, it could be adjusted or re-set back to the original fighter feature.

You're effectively playtesting a variant as you go, your player should apreciate that you're acomodating his character concept and not complain about the mechanics you're using to do so changing on him from time to time.