I have two units. One upflow and one downflow. One unit has stopped working and I have been told the current unit is big enough for the entire home. Can the two heating and air ducts be linked together and the upflow unit accommodate both parts of the house?

If 1 unit is big enough for the house then it is way over sized. You can hook them together, but it is hard to get air to go in 2 different directions without dampers and zoning.