Incendax said:
To the original poster: 4th Edition is a game of Paper, Rock, and Scissors. Throwing Brutes(rock) at the party generally calls for Defenders(paper) to counter. That means significantly higher AC to balance out the strong damage potential of the brutes.
As another poster suggested, I'd limit the number of Brutes you toss at the party, but Artillery, Lurkers, and Skirmishers should be fine pickings.
Indeed, this is something to really pay attention to. If there is no controller in the party, shy away from too many minions. If there is no defender in the party, shy away from Brutes. If there is no leader in the party, shy away from controllers. If there is no striker in the party, shy away from artillary positioned in hard to reach places. This is all spelled out in the DMG, Building a Party section.
Also the DMG clearly states, when designing an encounter, it is the DM's job to make sure the party has the tools to deal with it. For instance, if a creature is 4 levels above the party's level, they can typically handle it fine, but if said creature has some strong regeneration capabilities, which negates all the damage done to it, they will be hard pressed to survive that fight.
This encounter with the 2 firebeetles falls in that category of too hard of an encounter, especially when you look at the party composition. Between the 2 firebeetles, they have the ability to drop one character before he can act. For a party of 5, dropping one character would be a 20% loss, and fluke rolls are less likely to happen, when they are spread against 5 characters. For a party of 2, dropping one character is a 50% loss of resources. This is way too dangerous.
Running a game for 2 players requires a lot of finesse, and adventures should carefully be designed with the roles of the characters in mind. A leader and a striker is actually the recommended composition for a party of two (though I would prefer Defender and Leader). You just have to use fewer brutes, more skirmishers, fewer minions. Also create reasons for an NPC to travel with the party, as often as is feasible. They might have a Ranger guide who helps them through a forest. They might have a Rogue trapfinder, helping them through a dungeon. They might have a Fighter, helping them guard a caravan. They might have a Wizard who hires them as helpers to clean up a bug infestation. The role of the third wheel doesn't really matter much since you want to focus the adventure around the two players anyway, but the NPC will help you solve a lot of otherwise sticky situations.