more realms under the same external federation

If i have more then 1 domain, is it possible to add in the same realm under the same EXTERNAL FEDERATION? or do i have to creare one provider "external federation" for each different domain?

i mean, if i have for example and, do i have to add two external federations (under Identity and Authntication) or can i add in the same external federation provider, unders the field REALM for example with this sintax:; ? And in this way add other different domain, for example;; ? (and so on)

thank you