I've found that two characters is actually the worst set up for exp.

When you have a group of 3 or more people, you get the exp for the kill, but you also get a 'group bonus'. Just check your combat log. It works very similar to rested exp. The most group experience you get is with 5. Raiding with 6+ will kill your exp.

Basically, with 5 characters, you get less experience per character, but you actually get more total exp per kill (take the sum of all 5 characters).