You cannot go by what the upstairs thermostat is wired. Especially if it controls a separate unit. What matters is how the furnace that the downstairs thermostat is going to connect to.
As I stated before, you need to go to the furnace that the downstairs thermostat is going to connect to, if you have two separate units in the house, and get the wire colors and what terminals they connect to. That is what you are going to use to connect the downstairs thermostat to the downstairs furnace.
You only need to worry about one system, so forget about the upstairs furnace and how it is wired to the upstairs thermostat.
As I stated before, you need to go to the furnace that the downstairs thermostat is going to connect to, if you have two separate units in the house, and get the wire colors and what terminals they connect to. That is what you are going to use to connect the downstairs thermostat to the downstairs furnace.
You only need to worry about one system, so forget about the upstairs furnace and how it is wired to the upstairs thermostat.