No dual-monitor support for OS 10.6.5 (Snow Leopard)?

I installed Snow Leopard on one disk drive in my Mac Pro, but upon rebooting to the OS 10.6.5 disk, I could only get a signal on one of my two monitors. I checked the connections and the monitor menus, and went back to running from my 10.13.6 disk, which worked fine, as it had before the Snow Leopard disk was used, and I can't think of a reason for the problem. The GPU is the stock Radeon 5770, and I recall using two monitors successfully in 2009-2010, when I would have been using Snow Leopard. What could be the problem? What am I missing?