No indeed. If you dual-class, the XP you already have accumulated in your character's first class (rounded down to the exact amount needed for the last gained level), is subtracted from the XP-cap for the second class. For instance, a fighter with 95,347 XP, which means that he's level 7, duals to mage. His XP is set back to 64,000 - the amount where you reach level 7. The maximum XP he can now gather for his mage-class is 161,000 - 64,000 = 97,000. So he can reach level 8 as a mage.
So if you've already hit the XP cap, in this example the fighter would be level 8, his XP would be set back to 125,000 and the max XP for him as a mage would be 161,000 - 125,000 = 36,000. Max level he can reach as a mage would be 5. Which means that he would never get back the abilities he had as a fighter. Unless you would import him into SoA, of course.
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