Usually the motorised valves are physically next to each other, so if you were to connect the upstairs valve to the downstairs valve wiring terminals they will both open together.
Just a word of caution though. As you already have two heating zones, I'm assuming your property is fairly new (Post 2010) as the two zone arrangement is now a requirement of Part L of the building regulations. So, converting it to one zone would put it in contravention of the regulations. Although no one will come and inspect of course, but you may find that should you sell the property and the survey highlights it, you may have to reinstate it at your cost. Also, you will likely find that the upstairs room where the room thermostat is located doesn't have a TRV on the radiator, which may mean that that room will get overheated without a TRV if the control provided by the room thermostat was removed.
Just a word of caution though. As you already have two heating zones, I'm assuming your property is fairly new (Post 2010) as the two zone arrangement is now a requirement of Part L of the building regulations. So, converting it to one zone would put it in contravention of the regulations. Although no one will come and inspect of course, but you may find that should you sell the property and the survey highlights it, you may have to reinstate it at your cost. Also, you will likely find that the upstairs room where the room thermostat is located doesn't have a TRV on the radiator, which may mean that that room will get overheated without a TRV if the control provided by the room thermostat was removed.