Is there an RDP client that can work with a dual monitor setup?

You can run rdesktop -f or run rdesktop -f <width>x<height>.

-f

Enable fullscreen mode. This overrides the window manager and causes the rdesktop window to fully cover the current screen. Fullscreen mode can be toggled at any time using Ctrl-Alt-Enter.

-g Desktop geometry (WxH). If geometry is the special word "workarea", the geometry will be fetched from the extended window manager hints property _NET_WORKAREA, from the root window. The geometry can also be specified as a percentage of the whole screen, e.g. "-g 80%".

Recently I read about FreeRDP. I should support multimonitors, but I have no experiences with it.

You'll have to compile FreeRDP from sources because Multi-Monitor support is only available in versions >= 1.1.

Visit this page to get instructions and a list a build dependencies.

Then you'll be able to start a connection from a terminal like this:

xfreerdp /multimon /u:username /v:server_hostname:server_port