Multiple Digital Signatures on one pdf document for Adobe Acrobat Standard DC aren't valid?

I agree that your preparation process seems open to question: sign then clear then sign again? Why?

Multiple signatures are entirely viable and trustworthy. If the file is changed after signature A and B and before signature C and D then only C and D will be valid. But the legal status may be an entirely another matter; this depends on the laws in effect in the country, state or region where the document is prepared, and you need to consult a specialist lawyer.

There is a general rule: one person should NEVER sign more than once. We often see fatally flawed workflows where people believe they are "signing pages". Such misdirection may leave the signature invalid, since the signer was so badly informed.

I don't know whether the process by which you create documents with multiple signature fields is ok. If you provide an example with multiple signature fields as applied by your process, we can take a look and discuss that.

First of all, though: You call those signatures "Adobe Digital Signature" and discuss whether "Adobe Acrobat only allows for one legal signature." Those signatures are not owned or currently defined by Adobe, instead they are specified by ISO and other international and national standardization institutions. And Adobe in particular does not decide which signatures are legal, that is up to your applicable legislation and may or may not follow industry best practices.

Technically there are two main multi-signature-field cases to differentiate:

• Multiple signatures applied sequentially: Specifications commonly use this procedure for technically valid signatures.
• Multiple signature field widgets of a single signature value: This is discouraged by specifications; most ways to create such constructs actually are forbidden.

Concerning the legal value let me state that I'm not a lawyer. From experience, though, technically valid signatures usually also are legally valid as long certain additional, non PDF-specific requirements are fulfilled.

Be aware, though, that between applying signatures it may be allowed to apply certain other changes to a document, in particular form fill-ins, sometimes even adding/changing/removing arbitrary annotations. Here of course a signer only signed the document in the state he saw it and only is responsible for the contents at that time, not for changes after his signature.

A document can be prepared in a way that form fields a certain signer is responsible for are locked when he signs. Making use of field locking might be necessary in your use case.