Do you recall the high stat abilities in Rolemaster Companion III? A simular system could be divised based on "High levels in Profession".
As an eample, each profession would have three types of icks, A, B and C. At assigned levels (say, every five) they get to purchase a pick from the list of abilities. Multiple Atack could be one of them.
IF multiple attack were added as a fighter ability, I recommend the rule being something like for this: as a 100% action the fighter may attack two targets that are within 10' of each other. Each attack suffers a -40 OB mod. OB deduction for parry is subtracted from BOTH attacks.
At higher picks, the penalty can be decreased.