TL;DR: Dump party, ERASE and repeatedly scribe scrolls, buy repeat scrolls just for xp, win at dual classing. (Worthwhile even dualing to non-mage, and keeping one NPC mage in your party to scribe scrolls.)
If you dual class to mage, then dump your party and read scrolls, you can get to at least level 12 without having to do any combat or quests while underpowered from dual leveling.
For example, if you go to 11 cleric then dual to mage, you can instantly finish your dual class. With a 13 kensai, i'm not sure how much gold it'd cost to go all the way to 14 mage, but at the least you could skip any time as a low level mage.
I'm kind of a newbie but I've looked at some gameplay reports from expert players and this is one thing I don't see them doing.
The trick to this, which I haven't seen anyone mention, is to ERASE SPELLS from your spellbook so you can learn the same spell multiple times. And spend gold buying up duplicate scrolls in town. Also, obviously, use +int potions so you don't fail scribing. And to repeat: dump your party temporarily so you don't share xp.
If you are dual classing to something other than a mage, you would have to keep 1 other character in your party and have them read scrolls and split the XP. It'd still be worthwhile to get some levels to get your dual class started quickly.
In my current game, I saved all the scrolls I found (besides a few I wanted other people to learn and use), spent 30k gold on extra scrolls for xp, and turned in a 50k xp quest reward immediately after dual classing. That put me straight to level 12 mage with no downtime where I couldn't use cleric spells. A lot more than 30k gold could be invested in this strategy. You can get a LOT of scrolls for less than 100 gold per spell level, meaning every 10k gold gives more than 100,000 xp.
As a non-mage dual example, f you were dual classing from fighter to cleric and had an NPC mage read scrolls, you could still easily get up to level 9 cleric despite splitting the xp with someone else. And higher is achievable. (And you get to overlevel one NPC mage which is nice too).
This is for bg2. I don't know how well it'd work in IWD. It does not work very well in bg1. Scribing xp was multiplied by 100 between bg1 and bg2, which is not balanced.
On a related note, don't sell extra scrolls you get as loot. Just erase them and learn them (save them up before your dual class, after that just learn them with a full party present for convenience). Like there is that bandit encounter when traveling which always gives the same 3 scrolls (oracle, monster summoning II, spell thrust, if i remember right), and they won't sell for much, but you can get your party 12k xp by scribing them.
If you dual class to mage, then dump your party and read scrolls, you can get to at least level 12 without having to do any combat or quests while underpowered from dual leveling.
For example, if you go to 11 cleric then dual to mage, you can instantly finish your dual class. With a 13 kensai, i'm not sure how much gold it'd cost to go all the way to 14 mage, but at the least you could skip any time as a low level mage.
I'm kind of a newbie but I've looked at some gameplay reports from expert players and this is one thing I don't see them doing.
The trick to this, which I haven't seen anyone mention, is to ERASE SPELLS from your spellbook so you can learn the same spell multiple times. And spend gold buying up duplicate scrolls in town. Also, obviously, use +int potions so you don't fail scribing. And to repeat: dump your party temporarily so you don't share xp.
If you are dual classing to something other than a mage, you would have to keep 1 other character in your party and have them read scrolls and split the XP. It'd still be worthwhile to get some levels to get your dual class started quickly.
In my current game, I saved all the scrolls I found (besides a few I wanted other people to learn and use), spent 30k gold on extra scrolls for xp, and turned in a 50k xp quest reward immediately after dual classing. That put me straight to level 12 mage with no downtime where I couldn't use cleric spells. A lot more than 30k gold could be invested in this strategy. You can get a LOT of scrolls for less than 100 gold per spell level, meaning every 10k gold gives more than 100,000 xp.
As a non-mage dual example, f you were dual classing from fighter to cleric and had an NPC mage read scrolls, you could still easily get up to level 9 cleric despite splitting the xp with someone else. And higher is achievable. (And you get to overlevel one NPC mage which is nice too).
This is for bg2. I don't know how well it'd work in IWD. It does not work very well in bg1. Scribing xp was multiplied by 100 between bg1 and bg2, which is not balanced.
On a related note, don't sell extra scrolls you get as loot. Just erase them and learn them (save them up before your dual class, after that just learn them with a full party present for convenience). Like there is that bandit encounter when traveling which always gives the same 3 scrolls (oracle, monster summoning II, spell thrust, if i remember right), and they won't sell for much, but you can get your party 12k xp by scribing them.