I think this is fine as long as you only match pairs with pairs, light parties with light parties.
As it is I think they should make frontlines participation like Crystal, solo entry only.
You just can't compete with if a coordinated pair.
When you then add that some Modder will (if they haven't already) come up with a mod that backchannel coordinates auto targeting and attacks between two or more players, it would be unworkably devastating.
That said I see no cost to them allowing it. A league for solo entries, a league for pairs, a league for light parties.