You can't have multiple SAPs on a single Business Unit. But you can add multiple Private Domains, which can be used both as an additional sender domain, as well as a domain for your Cloud Pages.
For sending emails, SAP and Private Domain are equally good. They both offer the same degree of authentication (SPF/DKIM/DMARC), the few differences are:
- There can be only a single SAP on a Business Unit
- There can be multiple Private Domains on a Business Unit
- Only SAP provides link branding of links and images within emails
When it comes to Cloud Pages, both SAP and Private domain can provide branding of your Cloud Page URLs.
You should raise a support case to have your Private Domain configured. It normally requires your DNS records, for the subdomain you want to use, to be delegated. It also requires, that you have a Private Domain available in your contract.