Hi gpunx.
We are aware of the different behaviour on Yosemite, we’ve disabled the multi-monitor fullscreen to be compliant with the separate Spaces feature (no windows can be used across multiple displays). Even so we understand this fullscreen mode can still be useful even if the user wants to keep Displays separated, so we’re evaluating to add a new feature request to support OS X 10.10+ and span the session window across all the monitors regardless the Spaces preferences.
About the Microsoft RDP client and Dual Displays, what do you mean exactly? We’re currently working on this feature, may it be related to your request?
Giving the possibility to switch between monitors of the remote server
https://www.nomachine.com/FR11K02797