Wait a second: Does each side in the boarding combat get one attack per round, or one attack per combat unit? For example, if you have a boarding combat with each side having 5 combat units, is each side attacking 5 times (once with each combat unit) or is each side attacking only once?
Each combat unit gets one attack on one other combat unit. They cannot gang up on each other (this makes it pointless beaming a unit aboard, say, a Colonial Battleship).
The assumption is a bunch of isolated battles taking places in various locations about the ship. No more than one unit can fight another.
The advantage of having lots of units is that once a boarding unit has beaten one of your units, it has to defeat another. But it has to do so sequentially.