Hey Nicole,
We have a great article here:
https://community.marketo.com/MarketoResource?id=kA650000000GtnhCAC which goes over how to manage multiple landing page domains. Basically, you set up CNAMEs for all your different brands and add them as Domain Aliases. Then, when sending out emails to a certain brand's leads, you just create the emails to have the correct domain alias in all the links.
Currently, only one Branding Domain is possible at a time, regardless of how many workspaces are present.
For DKIM, you will just want to active it on every domain that appears in the From address of your emails.
Best,
Will