Soooo, maybe I missed something really obvious here.
Case is, having a open planned living/dining room, which have heating cables divided in two zones.
However, want to control it with just one nest thermostat.
Is this possible with an smartapp and a fibaro relay?
Or do I need two fibaro relays, which one is connected to the heat link, and with the lighting smartapp turns the other on whenever the first one turns on?
What else is the nest connected to?
If the fibaro’s actually control the on off of the heat, you can use nest manager to get nest integrated into SmartThings, and then write an automation to turn on or off your fibaro’s
The thing you need to consider in all this is do you want ST as the “control system” for your heat being on/off? i.e. what happens if ST has a problem, can you live with your heat staying on or staying off without any other “fail safe” in place?
There are many comments in these forums on reliability of ST, in general it is for convenience rather that “core safety”.
Today with other thermostats, in the end the thermostat is controlling the device, and ST is offering the thermostat settings. If ST were offline, the thermostat still does its job.
If you configured your system that ST is the control of the heat, and the thermostat is a sensor, this may be too much trust in ST…
If you have two thermostats in the room, each controlling a portion of the heat, Nest manager does have built in automations to mirror settings from a primary thermostat to others, so you can setup that changes made to what you designate as the primary mirrors.
You can also setup a built-in automation in Nest Manager that would use both temp sensors to figure out the “room” temp along with the above mirroring.
In this case, the thermostats each control a portion of the heat (core safety is in the thermostat), and NM and its built in automations gives you the single zone operations. In this case you would not need Fibaro, and you would not be relying on ST for core safety.
Maybe I’m putting to much trust in smartthings, haha:laughing:
However, already having a nest in the kitchen controlling the heat there, it kinda feels like overkill to have another couple of meters from it (dining room) .
However, in the living room, I can put up a nest.
But the issue still remains, because the nest can’t handle the wattage of the heated floors, I’ll still be using a fibaro relay.
So for the dining room I’ll need a “app” that lets me turn on the relay when nest is set to heating and also turns it of when the nest stops heating.
Would be great in the living room also because it would have saved me from some cables, but I can also use the relay hooked directly to the nest heat link. (yes, I live in Europe) 
Does anyone know of anything or an specific app that gives me the possibility?