This is a compromise which isn't quite the dual class that you're suggesting, but I've done it in my latest journey and it has been great fun...fun which hasn't finished just yet.
You could always look into starting out as a Druid from BG1 then import to BG2 and take the Avenger kit. This way, you obtain particular spells from the early level mage grimoire...I'll leave it up to you to investigate the details, but it does have its advantages. [img]smile.gif[/img] (A near fatal addiction to Chromatic orb spells for instance....)