Unfortunately, Radmin is not compatible with Remote Desktop. It is not recommended to use more than one remote access tools on the same computer. Moreover, there's no need to use Remote Desktop connection when Radmin connection is available, since Radmin offers functionality similar to Remote Desktop plus a lot of additional features.
I had to throw my two cents worth in for this one.
I agree that RAdmin is far better than RDP for connecting to a workstation. However, RDP is far more efficient and effective when you have multiple users connecting to a Terminal Server (as I suspect may be Mr. Sanchez's case).
Roman is right, though, in that RAdmin will not connect to an RDP session, only to the console session. Your Terminal Server configuration does have settings for monitoring and/or taking over one RDP session from another.
RDP and RAdmin can peacefully coexist. I frequently use them together for a variety of tasks: connecting to an RDP session, then using RAdmin to connect to workstations from the server or to the server console for those tasks that can't be done from an RDP session. My favorite is setting up RDP environment settings for certain users so that when they log in, they immediately get an RAdmin session to their workstation, allowing them to work from home without getting any real access to the Terminal Server.
I don't know that the possibilities are endless, but there certainly are a lot of them. And they're worth exploring for those of us that do a lot of remote access support.
Remember, if you're not having fun, you're not doing it right!