This is a shameful post, but I used the following tactics to beat the twins.
In the first section (their throne room I think), I tanked everyone up on haste and then pulverized their minions... I totally ignored the twins. When the minions were all dead, I switched everyone over to missile weapons.
I had over 800000 gold pieces when I last spoke to that sneaky drow on level 4 of the severed hand... I just dumped all the eqipment I didn't need and bought 800000 bucks worth of +5 bolts, arrows, bullets and uh... I cant rememeber if there were any +5 darts... but if there were I bought em.
Back to the throne room... I then took one of my fighters (a half orc barbarian with boots of speed) and got him to act as a decoy for Isair and Madae. They dutifully followed him round the throne room while he just ran away keeping out of range of their weapons. The rest of my party just hid in a corner of the throne room doing nothing. When Isair and madea's annoying buff spells finally ran out after 10 minutes of real time or so of just running around like a nonce, I brought in my drow ranger and the two of them proceeded to run away from the twins, turn round and fire at them with their +5 missile weapons.... run away, turn round and fire, repeat ad nauseam. The twins finally retreated to the pool room. I had taken zero damage... although i felt so damn cheap.
I went to the pool room (gnashing my teeth when my party just 'appeared' in the middle of the pool room... yeah right, like my characters are just gonna walk into the middle of the room... cheeap, cheap). I then ran away again, and peppered those annoying Slayer Knights with some leftover +4 arrows, corrosive bullets etc. I then had 4 memebers of my party go and hide in the throne room again while my barbarian and ranger did the whole run away, shoot, run away shoot, etc again.
I only used tow characters to do this because of the terrible AI in IWDII... man, when you're using loads of characters and they bump into each other, they always end up going the wrong way and 'grrr' I just decided to take the long way and use two characters only. It took another ten minutes or so, but I finally peppered that annoying Isair into Limbo.
OK, it worked... but I felt (and feel) cheap. I've never had to just be some lame on any Infinity Engine game before, but those twins were just too damn buff. I mean, Isair's sword! WTF was that all about? One hit - just one hit- and BANG your character is stunned for x rounds and gets chopped to pieces... man, that weapon is just too rock hard, which is why I went for missile lameness. I had a 16th level mage who did basically zippo in the final battle. What a shame :'(
Just my two cents

Looking forward to Temple of Elemental Evil (which is out in October in the UK wooohoo)