I've played in plenty of games with multiple defenders.
It's really not as big of a deal as it sounds.
The Big guys with swords (or skinny guys with Arcane blades) get up front, choose their respective opponents, and the fight goes on.
Rarely do the two end up trying to mark the same guy.
If they do, it's usually at the start of their turn and only to get an extra effect from a power.
If it turns into a huge fight, have the bad guys kill one of the defenders and make them play a different role.