View a 2-frame ds9 to view two images.

ds9_2f_markII  
Usage: ds9_2f_markII a.fits b.fits zscale
arg1 - FITS image #1
arg2 - FITS image #2
arg3 - method of image scaling for greyscale (minmax,zscale) 

You can get more detailed information with: 
ds9_2f_markII --help