In Magento 2, it is possible to extend themes by inheritance.
Is it possible to have more than 1 parent theme?
No, you can't have multiple parents. That would make inheritance very messy. If both parents had the same template, which would take precedence?
As Asish said, all you can do is make use of multiple-level inheritance. Theme A can't extend Theme B and Theme C simultaneously, but Theme A can extend Theme B which extends Theme C.
Theme A > Theme B > Theme C
Does not work:
Theme A > Theme B +
Theme A > Theme C