Has_many :through =>?

I am having trouble setting up a relationship using the through option.
Say you have the follow:

A company has many company branches and facilities. A user must belong
to either a company branch or a facility.

I just want to set up a relationship for the company that is through
both the company branches and facilities. How can I accomplish something
similar to:

has_many :users, :through => [:company_branches, :facilities]

Thanks for your help.