rdp span to dual monitor not woking in win 8

I recently bought windows 8, I had windows 7 previously , for rdp to my work pc I use this comand
mstsc /v:10.20.53.2 /span
this will open remote computer in both my monitors, this is does not happen in windows 8 ,
plz advice ,

According to mstsc /?

"/span -- Matches the remote desktop width and height with the local virtual desktop, spanning across multiple monitors, if necessary. To span across monitors, the monitors must be arranged to form a rectangle".

Are your monitors, perhaps not aligned properly?

You may also find use with /multimon