I don't think it's possible to do it in one frame, try separating it in two fsm's and see if it works out for you.
FSM 1:
State 1: get key a, go to state 2
State 2: get key b, go to state 3
FSM 2:
State 1: get key b, go to state 2
State 2: get key a, go to state 3
Since it's basically impossible to press two keys at the same time, two fsm's will make sure the combo is detected regardless of which key you pressed first.
or you can try it out in one fsm too
state 1: get a, if pressed go to state 2, get b, if pressed go to state 3
state 2: check if b pressed, if it is go to state 4
state 3: check if a pressed, if it is go to state 5