Can you have two different iPhone accounts backed up on the same cloud acct?

Sorry, your question does not make sense.

There is no such thing as an iPhone account. There are Apple IDs / iCloud accounts. Commonly the Apple ID is the same as the iCloud account, and as such the associated devices back up to the iCloud storage space for the Apple ID being used.

If the iPhones are sharing an Apple ID for iCloud, then they will back up to the same iCloud storage space. However sharing an iCloud account is not recommended.

Each iPhone if used by a different person should have its own Apple ID / iCloud account.

You can however share a storage subscription between different Apple IDs, so devices signed in to different Apple IDs can backup to the same purchased storage.

See here: link-> Share iCloud+ with your family - Apple Support