[SixOfSpades]

I'm agreeing with most of the stuff here......

Backstabbing Thieves should invest in Single Weapon, not Sword & Shield. S&S is practically worthless for many characters, as it only boots your AC against Missile attacks. Your Tank and your Backstabbing Thief don't need it, as they're always within melee range when they're fighting. Your Thief shouldn't use a Shield at all, because with 1 weapon and a star in Single weapon will let you Critical Hit (and we all LOVE to see that message on our Backstab rolls, don't we?) with a 19 as well as a 20.

Katanas are good to specialize in early on, but not before Long Swords. The Celestial Fury is a damned good weapon, but you'll need some levels under your belt if you want to survive the battle to get it. Also, some of the Long Swords later on in the game are just plain better. (I'm thinking ToB here.)

Thieving Skills: It's nice to have both/all Thieves in your party have close to 80% Set Traps, because being able to set more of those nasty little suckers never hurts. Pass around the Mercykiller Ring to get everyone's percentage up to 100%, which is the maximum that makes any difference (I think).
Only one of your Thieves should do Detect Illusion. (I'm playing a bit with Imoen: I turned her into a pure Swashbuckler, and will Dual her at Level 15 or 20. I put all her points into Find Traps and Illusions, and gave her the Thief(Adventurer) AI Script. I'm going to see if it gives her permanent True Sight.
I like to have one Thief for fightin' (wears the Mercykiller Ring/Boots of Elvenkind/etc) and one for stealin' (carries a stash of Potions of Master Thievery).

P.S. Read Nifft's P.S. again. Little tricks like that is exactly what I do when I want to plant a Backstab (I always do the "talk to the person to get them facing the way you want" ploy when facing the Lich in the Crooked Crane.)