2011 2.15 iMac & 2009 21.5 imac dual monitor?

[trackbikes]

I have my old 21.5 2009 core 2 duo imac plus my new 21.5 imac.

Having trouble selling the old one here in spain so I wondered if it's possible to just use it as a 2nd monitor?

If so what cable would I need?

 

[simsaladimbamba]

Only the 27" iMac can be used as external display.
To use the 21.5" iMac as display, try AirDisplay.