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.

Moderate complexity - several routes or missing facts

An application may be made by a person "either alone or jointly with any other person" (section 6(2)).
Once granted to two or more persons, each holds an equal undivided share unless an agreement says otherwise (section 50(1)). If your intended shares are not equal, say so in writing.
Unless an agreement says otherwise, one co-owner cannot grant a licence or assign a share without the others' consent (section 50(3)). Decide now who can license, to whom and on what basis.
Cover who pays filing and renewal fees, who instructs the agent, who may sue and share costs, and what happens if someone leaves, dies or is wound up.
Any assignment of a patent or a share must be in writing and duly executed (section 68).

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.