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

Unless an agreement says otherwise, each registered co-owner may exercise the section 48 rights for its own benefit without accounting to the others (section 50(2)).
If a co-owner sells a patented article, the buyer and those claiming through the buyer can deal with it as if a sole patentee had sold it (section 50(4)).
That does not let a co-owner license or assign alone (section 50(3)).
If you want profit-sharing, royalties or exclusivity between you, write it down. The default rule does not provide it.

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.