Three channel hack?

Hi

I have a question on whether I can create a hack to replicate a three channel set up. A long pre-amble to the proposed hack at the end which I am looking to get feedback on:

SET UP BEFORE INSTALLING TADO:

I have two zone heating and one hot water. The two zones were controlled by 2 controllers as follows:

a. CH1 - upstairs which used to be controlled by a wire thermo and has its own valve

b. CH2 - kitchen underfloor heating which was controlled by a wireless thermostat and has its own valve.

MY GOAL BY MOVING TO TADO:

My plan WAS to use TRVs in the upstairs (about 12 radiators) and set up to two extension kits to manage each CH separately. Having bought everything and installed most of it up to the second extension kit when, I have found out Tado does not support two extension kits on the same account (pretty silly, btw). After a lot of cursing, I did some research.

CURRENT INSTALLATION (NOT HAPPY):

Currently I have connected both CH to slot four (CH on) which means that they both work in tandem. My issue with this is that if the underfloor heating thermo tries turns the heating off while the TRVs turn the heating on, then the underfloor heating will continue as the valves will remain open. This is not ideal. As a hack, I hooked in the wired thermostat (an old dial system) between the extension kit and valves on CH2 as a way to act a manual switch. Pretty clunky.

ALTERNATIVE OPTION:

Another thread recommends to connect each CH to the live (i.e. always on) and use thermostats as a relay switch. I am considering a variant by connecting CH1 to live in on the extension kit and then the TRVs will draw upon the heating whenever they switch on. I am a bit hesitant because (a) some radiators dont have TRVs so there is some manual work to turn things on / off and (b) the pump will going on all the time. Even if all the radiators are off, then the boiler will still be working and circulating hot water which will cost money (some estimates are £3 a day!). 

ALTERNATIVE HACK - WANT FEEDBACK

The other idea I had would be to replace the existing wired thermostat that I have currently got with a Tado smart thermo so it will be a switch between the valves of CH2 and the extension kit (like I explained). That means CH2 will only work if both the thermos make a request. The issue I face is that the wired thermo goes to a different part of the house and not the kitchen. I figured that I could add the second Smart Thermo to the kitchen in the settings (even though it is not physically there) and make the smart thermostat that is physically in the kitchen as the 'measuring device' in the settings. My hope is that when the smart thermo that is physically in the kitchen drops triggers a call for heat, it also forces the wired smart thermo to do the same which will complete the circuit to open the valves for CH2.

Does anyone know if that would actually work?