SOLVED: tado setup for one boiler with two heating circuits (UFH and radiators)
Slightly odd one perhaps but as I spent quite a bit of time researching whether tado could work for my home's heating setup and didn't find any clear answers I wanted to post for others to see that it is possible -- and how!
Original setup: one boiler (Vaillant EcoTec Plus 838 + two heating circuits, one for the underfloor heating in our kitchen and one for the radiators around the house + two Heatmiser RF-Switches linked to two Neo Airs (i.e. one controlling the UFH and one the radiators)
The issue: According to what I'd read the tado system wouldn't be able to handle two wireless receivers (to replace the two RF-Switches controlling the UFH and heating separately) on the same system with one internet bridge. Additionally it's a relatively large Victorian house in the UK, with pretty poor insulation and some rooms quite far apart, where we'd want to be able to heat/control each room separately including the UFH in the kitchen. On top of this, although fairly tech-savvy, I've never touched anything heating-related nor am I an electrician of any sorts.
The solution: in short, we replaced one RF-Switch with a smart thermostat to control the UFH and the other one with a wireless receiver which is then linked to multiple TRVs across the various rooms (all on one system/internet bridge). The installation was actually surprisingly easy by admittedly winging it a bit... it wasn't difficult to replicate the Heatmiser's wiring with the wireless receiver and the smart thermostat by essentially matching the symbols/replicating the same wiring logic on both. Key also was point (3) below which was a pre-existing setup we hadn't realised was already there but turned out to be at least as important as the tado product combination we used.
Just a couple other quirks to mention as additional details to our solution include (1) that the smart thermostat controlling the UFH is (along with the wireless receiver) in the cupboard housing the boiler, which means it get very warm in there anytime the heating is on for anything in the house which we solved by adding a wireless temperature sensor and assigning it to the same room as the smart thermostat so that the wireless temperature sensor measures the actually temperature the other side of the kitchen for the smart thermostat to then turn on/off the UFH heating valve and the boiler; (2) like with the UFH, the boiler will turn on if any of the TRVs require it, plus we added a wireless temperature sensor for a room where we cannot install a TRV on the radiator... the upshot is that radiator is always on and hot if any other room's TRV switches on the heating but it also turns on the heating even if only it and no other TRVs need the heating on; and (3) already with the original setup using the Heatmisers we saw the electrician/plumbers had used a terminal junction box to manage the connections between (i) the UFH value, (ii) the radiators value, (iii) the boiler, and (iv) the two RF-Switches which we've replaced as above. It's through the junction box that they had ensured that the boiler is turned out with either of the two RF-Switches (effectively an OR gate) while the switches also control the individual values separately. Not complicated just required a bit of logic to make sure switches control their respective value plus the boiler. Lastly also putting the internet bridge in the very middle of the house, as far away from the router as the short ethernet cable allows and using double-sided tape to keep it upright (next to the electrical wall socket) did actually help as one TRV in a room further away sometimes go offline before doing that.
Happy to provide any further info but hopefully this already helps others in similar situations where they want to control two heating circuits of under floor heating and radiators with one just the combi boiler.