SmartThings has always had issues with multi gang switches because it puts its own architecture layer over the top of the third-party standards, which means things often work great for a year or two, and then suddenly stop working, because smartthings has changed something on the backend without notice. There are many threads in the forum about these kinds of issues. 
If you can wait a few months, this is a device class where Matter should bring a lot of new inexpensive candidates that wonât require any custom code. They may be Wi-Fi over matter or thread over matter or zigbee connected to their own hub which then bridges them to matter, but they will all run locally and should be easy to install. And hopefully be removed from some of the historical issues with multi endpoint devices.
If you canât wait, I think there are two good options right now, but both may lead to a frustrating installation experience that requires a lot of extra work.
- the most popular for power users: get one of the many Tuya Zigbee switches (these are sold under almost a dozen different brand names, each with its own idiosyncrasies), and try it with Marianoâs custom driver. If that doesnât work, then youâll have to do some research to find the exact âfingerprintâ (in the Tuya world, even two devices that look identical and have the same model number may have different fingerprints if they have different versions of the firmware) and then ask him to add that fingerprint to the driver.
(EDGE Driver-Mc): Zigbee Switch Mc, Zigbee Switch Power Mc, Zigbee Multi Switch and Child Mc
Thatâs obviously a fragile solution, because at any point smartthings might change something that breaks the integration, and then everyone is depending on one community developer, Mariano, to fix things again. Also, Tuya devices may not have all the same safety certifications. Still, itâs a popular solution, because tuya devices have very cool features, and are very inexpensive.
- the alternative is to get one of the options that doesnât have its own proprietary code, like the Aeotec line of inwall zwave modules. I was going to link to their dimmer module, but everybody seems to be out of stock of those on the European/UK frequency, which may mean that a new model is about to come out.
There are some other brands, but most of those will require custom code, and then weâre back in that fragile situation, only at a much higher price point.
So⊠my suggestion would be to wait for matter if you can. Hopefully other people will contribute with comments about switches that they do have working in the UK right now.