Even though you could use any relay to do this, we can't recommed tadoº to be used in this way. Those two systems have a very different control logic and even if it were possible to wire both of them to the same relay in tadoº, the performance wouldn't be good at all.
Have a look at fitting a HomeKit compatible 3-pin mains socket like this
to your electric heating.
You could then define a HomeKit rule so that when the Tado calls for heat it turns on the mains socket and when it is satisfied it turns it off. (This approach certainly would work for an ordinary electric heater, not sure about underfloor heating.)
IFTTT might also do the job.
The EveHome socket does not support IFTTT it only supports HomeKit. The WiFiPlug supports both HomeKit and IFTTT.
The WiFiPlug seems to be UK only.
The Ikea Tradfri is available for multiple countries, it is a standard Zigbee device. It can be upgraded to support HomeKit but via a standard Zigbee compatible Smart Hub it could be linked to IFTTT, e.g. Samsung Smartthings.