I assume you have the downstairs switch wired up with no load attached. You can use SmartLighting for this by creating two automations:
- Control Switch 1, Mirror Behavior, Mirror Switch 2.
- Control Switch 2, Mirror Behavior, Mirror Switch 1.
This means that if you turn on switch 1, switch 2 also turns on, and vice versa; I believe it will also mirror dim levels but I’ve never tried that, maybe someone else can confirm.