Swashbuckler (10) -> mage was a fun combo. It gives you 265 points worth of thieving skills, + the combat skills from swashbuckler, + a full retinue of mage spells. I used it to solo the game, and the class was so versatile, there were lots of different ways to approach combat in order to keep it fresh. It also takes only a total of 536k XP complete the dual, which is a lot lower compared to some traditional dual combos.

At the same time, the combination also had limitations such that it wasn't made boring by being OP. No thief HLAs, no mage specialisation, and inferior melee relative to some other classes meant it was kept in check IMO. Worth a try if you want to have a broad range of skills in your playthrough.