If i remember correctly, a 'kit' class can never dual class. Not sure about it though. Have you checked the bg dual class faq on the main forum??
is nothing mentioned about an archer/mage. and you would probably need amazing stats to get a good one too.
If you wanna be 100% sure, you can always make an archer with the required high str, dex, wis AND int (
) and see if you can dual him.
Or wait for someone else to confirm my statement (or say i'm wrong)