First, your Pro-HD has not failed. If you had searched the forum before posting you would have found that you have the classic symptoms for a failed power supply. So all you need is to buy a new power adapter. And I would recommend buying the official one from Sling as a lot of third-party power supplies are not good enough.
Next, you can have as many Slingboxes connected as you like, at one stage I had 5 of them.
For your 350, if you have it all plugged in to the network and the network light is on and you start the latest Desktop Player and tap on the Slingbox Directory the new box should show up, with a Reconfigure button next to it.
Thanks for the great suggestions.
I did try searching the forums but since this is my first time visiting I was either looking in the wrong place (it is possible I was searching the slingbox support instead of the forums) or using the wrong search terms.
While trouble shooting I was hesitant to try the 350 power supply with the Pro-HD (for fear of killing the 'good' power supply) but after your suggestion I checked the two supplies and they appear to be identical so my Pro-HD is now almost back up. The Pro-HD lights now work as they should and I seem to be able to connect but it does not seem reliable and I have yet to get a remote picture up and running.
Thanks again and hopefully I can get these things back up and fully running soon.
TJC in PA