We co-own a patent or application. What can each of us do?

Co-ownership works on a default set of rules that an agreement can change. Start with what you are trying to do.

High complexity - significant legal consequences

Before grant, if joint applicants disagree on how the application should proceed, any party may apply to the Controller. After hearing everyone, the Controller may direct that it proceed in the name of one or more parties, regulate how it proceeds, or both (section 20(5); rule 36).
If one joint applicant made an assignment or agreement, the Controller needs the other joint applicants' consent before directing a change of applicant (section 20(2)).
Keep records of who contributed what and what each party agreed.

The honest answer usually needs someone to look at your actual disclosure, your timeline and the prior art. That is a conversation, not an article.