Hi there,
Please, go to the following page of the Online IDL Help:
http://www.exelisvis.com/...itor_Configurat.html
Then, look for this last section "Example: Multi-Monitor Window Positioning", it mentions an example code called "multimon_ex1.pro". Simply, go to your IDL Workbench prompt and type:
.EDIT multimon_ex1.pro
Try that out, I think this is what you are looking for.
Cheers,
Fernando