Centennial and Castle Rock are not only close together, but property offerings for both cities are listed in the Denver MLS. As a result, I would suggest that the same agent can help you in both those areas. Just be sure that you pick an agent who understands due diligence and works to gain knowledge in the areas you are exploring. No one can be an expert in every area, so when you help a buyer outside of your home territory, it is important to spend the time gaining some familiarity with the market. I operate out of Evergreen, live in Conifer, and have helped clients purchase a new build prior to construction in Castle Rock, another client buy an older home in Castle Rock and covered much of the Denver area in client referral real estate transactions, so I know it can be done, and it can be done well.
As far as Colorado Springs, the agent you work with may be able to recommend someone down there. Typically, an agent who refers a client to another agent gets a referral fee. It is always nice if you are working with an agent and end up somewhere else, if they are able to refer you so that they have some compensation for their efforts.
I hope that info helps.