Most combat systems in RPGs let each player have a go in turn with the enemy, first to go decided by some initiative mechanic.

Some systems, like that in Nexus: The Infinite City, give each player a number of initiative points to spend in a round. The character with the highest number of points always gets to go. Basic attacks cost 3 points and defences 1 point. Typically characters have about 10 points to spend and these are refreshed at the end of the round. This generally means you get two attacks and two defences a round with a couple left over for moving or something a bit fancy.

Even this system, because it gives each character fairly similar amounts of points, doesn't allow for the cinematic multiple blows, although if you say doubled the points, and thus the spread, it would. You might even mutiply the points by endurance and use them to count not only initiative but fatigue. The points would have to last the entire fight and falling to 0 means running out of puff.

So a character with 5 Speed and 5 endurance would get 25 points, one with 4 Speed and 4 Endurance would get 16 points. Without special moves, the faster character would get 5 attacks before the other could respond (25-5x3=10, 16-5=11) which is probably a bit much. The values need to be carefully selected, perhaps everyone has a base of 20 and attacks have a higher cost or there's a ripost attack which allows you to ripost out of sequence if you're attacked.