A contains domain should enable you to relate domains on an as-needed basis independent of parent-child relationships. (For more information, see the documentation topic Understanding domain separation.) However, if you click Edit on the Contains Domains related list on the Domains form, only child domains are displayed in the available slushbucket.
Steps to Reproduce
Create two or three child domains to the ACME domain.
Go to one of the child domain records.
Scroll down to the Contains Domains related list and click Edit.
Note that the slushbucket is empty.
Add a record directly by clicking New from domain_contains.list or domain_contains.do.
Or, make sure you have the domain_expands_role, allows you to use the Toggle Domain Scope UI Action to switch from the domain record scope (which is what is restricting access to just the children of that domain) back to the session scope, which will allow you to use the session picker to select any amount of domain visibility in the slushbucket.
Related Problem: PRB639707