Try running this command on both:
configure stacking-support stack-ports all selection alternate
You will need to reboot.
After the switches come back online, do a Show Stacking command, you should see the other switch. Once you see that other switch, then the easy stacking command will work. I believe the recommended stacking config is port 51 to port 52.
Depending on how far along in the stacking process you have gone, you may have to unconfigure stacking on both first.
then run
enable stacking-support
configure stacking-support stack-ports all selection alternate
reboot
Hope this helps.