Avoid multiple characters, it kills roleplaying. [1]
Defenders are the lynchpin of the party IMO, so as long as one of the two PC is a fighter or a paladin, you should be able to simply send any monster you like as long as their total XP is a match of their level (200 XP at level 1). [2]
If they have no defender, then some combination of monsters, even if they total 200 XP, will be too deadly.
You don't want to send a single Elite Brute against a ranger and a wizard, for example. Their odds of survival will become entirely dependant on the ability of the wizard to inflict a special condition on the monster every turn in order to hinder him. A few bad rolls and the Elite brute will squish the ranger who wasn't designed to hold the line against this kind of opposition and then the wizard has to run for his life. Very swingy scenario.