For reasons of backwards compatibility, I continue to have Dimension LE, Dimension Pro, Rapture LE, and Rapture installed as VST2s alongside Rapture Pro and Rapture Session installed as VST3s. I have no operational issues caused by this arrangement.
The only problem I have found is of my making, in that in a moment of housekeeping zealotry, I purged the .wav files from the Multisamples, where .flac replacements exist, completely forgetting that this would cause "multisample not found" probs in some of the older projects. In these cases, I tend to substitute-in Rapture Pro, rather than waste time tracking down and reloading the .wavs.
As an aside, I have no idea why Rapture Pro and Rapture Session appear only as VST3s in my set-up, as the VST2s appear to be installed, but are not detected by Sonar's plug-in scan. I decided just to live with that.