A great combination. Swashbuckler to Mage. This is a new character i just started for a freshly made new game. You need to reroll a few times, because 2 of the most useful mage spells, the wish ones, require high wisdom. No more than 16 constitution, and an 18 to strength, dexterity and intelligence, no being cheap with those. But there are those tomes, lum the mad's machine and the tears of bhaal, so it is not that hard to both get decent stats and plan ahead on how to raise them.
Most people dual at either 10, or 15. I personally favor 15, because at hitting this level, you get another set of the kit's bonuses (per 5 levels), more thief points to distribute, and still have enough xp remaining to hit 28 mage, which is ideal (because at this level you get max spell slots). And do not worry about xp, regaining your class and stuff; 2 is full of xp loopholes, or you can import export and repeat irenicus dungeon a few times until you are ready to go.
Thief mage is great for many reasons.
1) BG2 + ToB both lack an able thief party member, able to progress as a thief (yoshimo dies, the gnome is an illusionist too which means no necromancy and wasted xp...).
2) Your main would be a real waste if he is not able to cast arcane spells, as an arcane class (mage, wild mage, specialist mage etc). Some reasons? You get to keep your hard earned spells with new playthrough, you can summon familiar and use its tricks or normal play, and last but not least, you can cast your way into endless spellcasting without rest, or simulacrum yourself and have 5 main char copies summon 5 planetars.
3) A rare combo. As the fighter mage one, it is extremely versatile, powerful, and many spells and items are tailored exactly for a dual or multi, mage/thief and/or fighter/thief.
Backstabbing is worthless, at least for me. All serious enemies are immune to it. Regular enemies affected by it, are easily dispatched anyway. Plus, swash is like a fighter, with low AC and great damage/accuracy bonuses. Plus, being able to put 2 prof points in weapons, is serious stuff. Having 3 points in dual wield, and 2 in both katanas and scimitars (the highest damaging weapons in thief's repertoire, excluding the staff maybe), makes this little fella something like a fighter/mage/thief. But better, since you do not have to share xp between three classes, you have 1 kit with most of its benefits (15 is high level), and you still hit 28 mage, which means not loosing a single spell slot. Wear robes of vecna, an ioun stone and enjoy.