EXAMPLE
If youre using the cap remover
can you be a shapeshifter and dual to a mage???

No, you can only dual class your druid into a fighter. The only whay to have druid/mage is to edit your main character with Shadow Keeper.

And why you cannot dual to a mage?

You can only dual into allowed multiclassing combinations. And since druids gain their powers from mother nature herself, mages gain their powers just from their knowledge about the arcane arts. Their spellcasting is therefore of totally different nature.

Despite that would the combo mage-shapeshifter would indeed be a very powerful one ...