If your agency has more than one owner, you should have an agency partnership agreement. In addition to (or as part of) that document, you should also have a buy-sell agreement that specifies how partners can sell or otherwise dispose of their equity in the business.
Having such a document in place from the beginning makes any changes to the partnership structure down the road occur in a more orderly fashion. Without such a document to guide the process, things can very quickly become contentious.
As important as the document itself is, the conversations that you and your partners will have while putting it together will help you ensure your visions are aligned and that you are considering all eventual outcomes.