If you look up the specifications of the relays in the dual what is the current needed for both.
My only thought is that you could physically link the low voltage control to both relays so if one was turned on the other was also unavoidably turned on. You could create a programmatic way to do this, using IFTTT or new firmware, but I wouldn’t trust it.
Does the heater really need 16A though? It is normally safest to have a safety margin and use a larger relay that is absolutely needed. My experience of immersion heaters is that they have a sticker saying how many kW they are. My current one is 3kW and has a 15A fuse. I measured mine and, at my voltage of 240V, it is 3.3kW. I suspect in continental Europe it would be 3 kW.
You could just buy a separate 20+ Amp relay and operate it by one of the relays in the dual. You’d need a separate enclosure for it. That might be the easiest way to do it and would allow you to keep the Sonoff warranty intact.
I'm still wondering if it is possible to connect to both the gangs of the switch I have, and thus receive the overall 16A?
10 days ago
The two relays are likely to have similar resistances so the current is likely to be roughly evenly split, but you need a definitive way to ensure both open together.
I still like a margin of safety between recommended max and actual usage. If you actually need 16A I’d want to use a bigger relay.