[Aerich]

Iirc, backstab is x3 at lvl 4-8, x4 at 9-12, and x5 at 13+. As MG points out, a T/C doesn't do much backstabbing, anyway. Sneak attacks, though - make sure you have sneak attack enabled on the console. A sneak attack is a flanking attack that doesn't require suprise - e.g. the thief is not targeted, attacks an enemy from behind or the side, and doesn't have to be stealthed. With a low DC thief level, the extra damage won't be much, but it's more useful than backstab.

I would dual soon, maybe at lvl 7 - you should be able to get a lvl 8 cleric in time for DE lvl 5, although you may have to 'camp' for XP on DE lvl 3 for a bit. I haven't played on core difficulty for a while, so my XP estimations are a little out of kilter.

The key skill for a thief dualled to cleric is trap removal. Pickpockets is fairly useless in the game beyond Kuldahar, locks can be bashed or Knocked, and the cleric can use the Detect Traps spell. I would recommend having at least 80%-90% in trap removal before dualling. It is the only skill that cannot be replicated by your party (aside from PP, which isn't necessary if you already stripped Kuldahar). You may take a few nasty hits from traps on DE lvl 3, as your mage caster cannot use Mirror Image (necromancy opposition school). A necromancer instead of a general mage or bard makes a good trap removal score even more critical for your party.