Are you sure this is necessary? My understanding was that the CRs and XP allocation for encounters are based on 3.5 rules, which assume that you're running with a party of 4, so that split amount is the amount of XP you should be getting. Running with a smaller party is a bonus for doing a more difficult task, not the other way around. This is what the DM Guide says about it:
When the party defeats monsters, you award the characters experience points (XP). The more dangerous the monsters, compared to the party’s level, the more XP the characters earn. The PCs split the XP between themselves, and each character increases in level as his or her personal XP total increases.
The normal-sized group is around four players (with the DM as the fifth person). However, some groups are as small as two players, and others as large as eight or more. (Very large groups sometime use a nonplayer assistant who helps manage player actions, rules referencing, and NPCs to help the DM keep from getting bogged down.)
Now, I'm not entirely sure if NWN2's XP is the same as the actual rules, though I know that there tends to be a lot more combat in NWN2 than you'd get in the normal rules, so you're leveling up faster anyway.