Since I couldn't find this information online - specifically, how it handles dual- and multi-class characters - I decided to do some research.
Black Pits caps at level 10, not at an XP level. This is 160k for rogues/bards, 250k for mages/sorcerers, 450k for clerics/monks, 500k for fighters/barbarians, 125k for druids and 600k for paladins/rangers. It is actually capped at either level 10 or 161k xp (cap for the BG campaign), whichever is higher, though this doesn't really make a difference.
For multiclasses, xp is still split evenly between classes and the max xp is equal to that of the class that requires the most. Anything with fighter in it, for example, would be able to gain up to 500k xp. (edit: cleric/thief and cleric/mage are currently capped at 250k xp, though this may be a typo.)
For dual-classes, max combined xp is that of the second class. So a thief dualled to fighter would be able to have a total of 500k, while a fighter dualled to thief could have up to 161k.
This has some interesting ramifications. For example, a fighter/thief would end at fighter 9/thief 11, while a straight thief could only reach level 10. Similarly, fighter/druids could gain level 11 in druid, thus gaining access to level 6 spells and being more druid than a druid could ever be. (edit: this seems to be incorrect. Druids are capped at level 11 for whatever reason.)
This also means that each of the triple-classes would end at 166,666 xp in each of their classes - more than the xp cap of the main campaign.
Black Pits caps at level 10, not at an XP level. This is 160k for rogues/bards, 250k for mages/sorcerers, 450k for clerics/monks, 500k for fighters/barbarians, 125k for druids and 600k for paladins/rangers. It is actually capped at either level 10 or 161k xp (cap for the BG campaign), whichever is higher, though this doesn't really make a difference.
For multiclasses, xp is still split evenly between classes and the max xp is equal to that of the class that requires the most. Anything with fighter in it, for example, would be able to gain up to 500k xp. (edit: cleric/thief and cleric/mage are currently capped at 250k xp, though this may be a typo.)
For dual-classes, max combined xp is that of the second class. So a thief dualled to fighter would be able to have a total of 500k, while a fighter dualled to thief could have up to 161k.
This has some interesting ramifications. For example, a fighter/thief would end at fighter 9/thief 11, while a straight thief could only reach level 10. Similarly, fighter/druids could gain level 11 in druid, thus gaining access to level 6 spells and being more druid than a druid could ever be. (edit: this seems to be incorrect. Druids are capped at level 11 for whatever reason.)
This also means that each of the triple-classes would end at 166,666 xp in each of their classes - more than the xp cap of the main campaign.
