Fun Fact: These Beholders are under the control of a Hive Mother. Otherwise, they'd be attacking each other for not being the same sort of Beholder!
It turns out that after all my musings about placing treasure in my dungeons, I didn't read the 'Lamentations of the Flame Princess' guidelines for setting up treasure correctly! Once you have calculated how much XP you want to place in the dungeon, you don't convert it all directly into treasure... Monsters are part of the equation! The DM should also count the total XP value of all the monsters in there. In fact I THINK I remember that the guy who writes the Henchman Abuse blog does that. (I could be wrong!)
This will require even more prep time but I do see the merit in it.