2 External monitors on 21,5 iMac Late 2009?
The late 2009 iMac uses Mini DisplayPort and not Thunderbolt, although they appear similar and share the same connector. The mDP technology included on the 2009 iMac does not support multiple displays.

In short, no. You will only be able to actually drive one monitor from the iMac, barring using a splitter. Such devices do exist but may not be able to provide you with a full dual-desktop experience (such as mirroring only, lower resolution, maximum display size limits, etc).

Source on Specs