I'd suggest not being grouped. Then have the lower level player 'tag' the mob e.g. by putting an instant cast on them or a DOT. Once that starts doing damage, the nameplate of the mob will turn grey for the higher level player. At this point the lower level will be the one who'll be credited with the XP for the kill, Now have the higher level AoE them down. That way there should be no XP penalty and fast kills.
I'm sure I remember people using this approach to get the fastest time to the level cap, by tagging things and having an army of friends / guild members burning them down. Link to a discussion of this: http://dwarfpriest.com/2008/04/30/mythbusting-1-70-in-a-day/
On March 19th, the German player Tosog
broke all of Athene’s supposed
records. He reached 70 in 1 day 4
hours and 20 minutes (video here of
him hitting 70).
He admitted to being boosted by other
players to reach 70 so quickly (he had
higher level players (not grouped with
him) kill off mobs he tagged). This
was a respectable coordinated effort
between multiple players.
It was considered a borderline exploit in the 'fastest to the cap' race I think.