Is there a way I can arrange for both visas using a travel agency?

For your HK visa, you need post your applications to the Immigration Department of the HKSAR. Its address is Floor 6, Immigration Tower, No.7 Gloucester Road, Wan Chai, Hong Kong. It takes about four weeks to get it. For mainland China one, you can assign an agency to handle it for you. But I don't think you can apply for them at the same time because you just have one passport.