It's possible to highlight the two images and press the C key to compare them. But that will switch you to the library module. There isn't any provision to display two images at the same time in the develop module.
Select the reference image and choose Window > Secondary Display > Loupe (Locked). Now go to the image you want to edit and start working. You can refer to the Secondary Display window as you work on the image you're editing,
If you have a second monitor attached, the entire second monitor can be the secondary display.
hey thanks jim
just a quick reply. im not sure if this is working correctly but while waiting for a response, i tried a couple more things. what i wanted to do, i was able to do in "secondary screen" mode...while remaining in the develop module. i havent completed processing the image but i was able to match the two images. i will work it some more to see if i can complete the action. again thanks for your help.