Two Zones: Both Heating Issue

Hi.

My Tado was installed yesterday. This is my configuration:

- An extension kit with wireless stat controlling my downstairs zone and hot water
- A wired stat controlling my upstairs zone

When I turn my (wired) upstairs stat on, only the upstairs radiators get hot (as they should do).

However, when I turn my (wireless) downstairs stat on, both my upstairs and downstairs radiators get hot.

The only thing I will note is that it seems that the upstairs radiators only get warm when the downstairs stat is on. Certainly not as hot as the downstairs radiators or if I was turn the heating on upstairs properly via the upstairs wired stat.

Any thoughts would be much appreciated.

Thanks
Nick