In Sonar X3e you were able to open multiple instances of Melodyne (I have Editor version) as a Region Fx, and have them available in the multi dock. I used this quite often to toggle between Melodyne clips on various tracks for editing (eg. especially useful for background vocal tracks) Now, in Sonar Platinum, on my system, when you open a second Region Fx - it just replaces the existing one in the multi dock . . . so only one at a time can be edited.
 
This is a change in behaviour since X3e.
I have tested by opening the same project in X3e, and when you open a second Melodyne Region Fx it appears next to the other one in the multi dock. (In fact, you can open many Region Fx, and they automatically populate in the multi dock)
 
In Sonar Platinum, you have to undock an existing docked Region Fx, then open another Region Fx, then re-dock the first Region Fx in order to have multiple instances in the dock. It's now an extra step, not sure why this would be intentional, and why this would have been changed ?
 
Anyone else notice this ?
 
 

This is by design - please read the documentation for plugin window recycling.
The default behavior of the app is to recycle plugin windows which is much more efficient for most workflows. 
 
There is no need to undock and dock if you do want multiple windows however. If you hold down CTRL while double clicking to open the region effect UI it will open a new instance in the multidock. CTRL open is a shortcut to switch the recycling behavior so it will pin the previous window and open a new instance. Take a look at the documentation - its explained there.

Noel, thank you so much for pointing me to the "plugin window recycling" documentation !
I've tested holding down CTRL key while launching Melodyne, and it works . . . glad to know it's an intended behaviour - just have to re-program myself to use the CTRL key to launch Region Fx.