The Microsoft RDP client handles this great, you can press 'command + 2' to toggle between full screen mode, then once it's off full-screen you can drag that RDP session window around (ie onto the second monitor) and then maximize (green button) the window so that it takes up the space it needs or use 'command + 2' again to make it full screen on the second monitor. Others bring up the the server desktop with the exact resolution of my MacBook LCD which is fantastic but then I can't figure out any way of moving that desktop to my secondary monitor.

Some servers go right off my screen when I log in with some huge resolution so I have to constantly scroll around with scroll bars to see all of the screen. I have endless frustrations with not being able to get the right resolution or even modify the resolution either through connection properties/options or from Display Properties within the session. Screen resolution/ mobility for published desktops. Works fine in Windows client but on the Mac you have to hit Num Lock (on a Windows keyboard) or Clear (on a Mac keyboard) to activate the number pad every time you log in. Num Lock status not passing through to the 2X session. Parallels Client (formerly 2X RDP Client), when connected to Parallels Remote Application Server, provides secure access to business applications, virtual desktops, and data from your device. Unfortunately it still hasn't fixed some other problems ie: It appears that the issue with the time zone redirection has been corrected with the latest version of the Mac Client (v7.3.768).

2x rdp client for mac