I'm all for adjusting things on the DM side. If the players don't want to play a leader that's fine by me, I'll adjust. I can reduce monster damage and/or use more minions and they don't need a healer. For a tough fight I may have terrain elements or battle stages that trigger surges. Terrain elements may include rune stones that give regeneration, or healing fonts that provide healing. Staged fights may see a small recess during which the BBEG makes his declaration of impending victory, while 1-2 healing surges may be spent and 1-2 encounter powers may be recovered. In an arena fight, a PC might use a minor action to make some sort of charisma check to impress the crowd, and the crowd cheers in encouragement, allowing the PC to spend a healing surge, maybe with a bonus.
Action economy can get really tight with 2 PC's, so I would tend not to use any monsters that daze, and would use prone sparingly. They can each pick up an emergency heal through a feat, item, or power, and they are set. But even in large groups, you want parties to have a couple emergency heals (or self heal) as it is, so this isn't terribly out of the norm.