Level 7 or level 13 are the usual choices for the extra half attack per round.
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You can dual right at the start to get weapon specialization (bonus to hit and and damage and an extra half attack per round) and start with up to 98 (7*14) instead of 42 (7*6) hitpoints.
You can wait additional 36,000 XP to get 8 extra hitpoints for fighter 8.
Waiting another 125,000 XP for level 9 will add another 8 extra HP, +1 to hit and a weapon proficiency point. You can still reach the highest mage level within the SoA XP cap.
You won't get the next noteworthy benefit before level 13 (extra half attack), but it's only recommended for powergamers with ToB installed. Most of the game will be over before dualclassing is complete.