That would be fine, as only 17 INT are required to dual to mage,
safly you want to dual to thief so what you need is:
- to be human
- to be lvl2 or higher
- not to be of lawful alignment (would conflict with thief class)
- 15 Strenght
- 17 Dexterity
The tomes will be found fairly late in the game, so I hope you already got 17 DEX.