Topic: Add Mongoid to ActiveRecord Gem side by side to a Engine

I'm trying to figure out the best way to add Mongoid to a specific Rails Engine (forem). I tought about using the mattr_accessor to configure a different ORM on the engine. But how would you go about implementing the actual logic for Mongoid models in parallel with ActiveRecord models? I tought about going with orm_adapter and using some magic on the module level to have two separate classes for Mongoid/ActiveRecord. What do you think about that or does anybody have some experience with adding a distinct ORM solution to a project without breaking the existing one?