If you connect two Slingboxes to the same cable box / satellite receiver / DVR, then when both are in use, they will be restricted to watching the same content. If that's what you want and the PRO-HD is connected with component cables, then choose a model with component inputs, which you will connect to the component outputs of the PRO-HD.
If you want the remote viewers to be able to simultaneously watch different programs, you will need to get a second set-top box for the new Slingbox. In that setup, the two Slingboxes are separate installations and do not directly connect to each other. You can choose any model that is compatible with your video source.
For either of the above scenarios, if both Slingbox users are remote (Internet viewing), your upload speed will be shared by the two boxes. You will either have to accept a picture quality degradation when both boxes are in use, or pay for an Internet connection with higher upload speed.