Can I connect my two HVAC systems?
Connecting the 2 may make things worse. Trying to control 2 different systems I have done but interconnecting the ducts would need to be done on the supply and return with dampers to be able to adjust them. An 8" duct is not going to provide a lot of change over that distance. I use 6-8" ducts for single bedrooms with just a short feed from a trunk line. My concern would be the system efficiency could be reduced for example sending cool air upstairs to the 2nd unit could cause that evaporator to freeze up. But if dampers in the duct were added you could possibly improve the mix I would just be concerned about freeze ups.