Just want to know if it is possible to take a screenshot from my second monitor.
Yes it is possible, however it will depend what is on there as to the approach. If you want to capture the second screen regardless of what is shown there, then you might need to look down the road of mirror drivers and the like (or more traditional desktop capture approaches). If a fullscreen Direct3D window is being shown on the 2nd display then you might need to look at grabbing the image from Direct3D.
If the 2nd screen is showing the desktop, I don't think using the DirectX approach isn't going to work unless you are working with the DWM.
You may need to implement more than one approach to handle all the scenarios, but again we don't know what you are trying to do exactly.
Just a tip - capturing from the front buffer is always going to be slow, and capturing from the backbuffer from 3rd party apps is usually done through hooking (take a look at the Direct3DHook project in my signature).
Other approaches include a mirror display driver but I don't have much experience with these.