homeassistant

You could replace them with z-wave switches. The switches by default would control the respective lights they're wired to, but you could use scenes to control the other switch. For example, 2x up on the canister light switch turns on the pendant light (and not the canister lights, unless you want that, too).

I have similar stuff programmed with Home Assistant using Node-Red, but the normal automation stuff would work, too.

Home Assistant/Node-Red sees that Scene 2 (or whatever) has been called for, and then does whatever you want.

+1 for Shelly as I have or any other drop in relay, all the wiring you'll need to do is behind the switchplate, you can decouple the input switch from the relay output and have HA trigger either output based on some input conditions. My fav config is having two flips of the switch perform a different action

shelly relays will do exactly what you want. just wire them as disconnected switches. i do this to simulate 3-way switches, but it'll work just as well to swap circuit behavior.

you can use a homeassistant action if you’re already using HA, or you can have the shellys call each others web api when it senses the switch.

If you want to skip wiring all together, pick up two Aurora Dimmers and put Hue bulbs in all your sockets. It's not the cheapest or most privacy friendly if you're using their hub. But going this way, you can put the dimmer near the fixture you want to control, you have no wiring whatsoever, connection to HA, and you're completely up and running inside of 30 minutes.

AliExpress some ZigBee light switches. They arent connected to anything but through Openhab you use them to swtch ZigBee light switches that replace your existing switches.

A low-wiring way to do it would be to replace the bulbs with hue/similar bulbs, then just put a battery powered button in the location you want to have the controls. £10-ish for each button, plus however much the bulbs are.

Then just have the button set to toggle the lights on/off (you can also call different presets like dim etc by pressing and holding).
Then hass just directly sends the on/off commands to the bulbs.

As long as the sockets have the power from the original switches left on, any kind of wireless bulb and switch would work. The switches then control the bulbs, not the actual power circuit. Just make sure to get brands you know will work with HA if you plan to tie the switches into the system.

Install Enocean wireless, battery free switches. I've used them for many years, they work awsome. The switches can go anywhere you want (double side tape on any surface) or screw in as normal light switches. The act of pushing the button generates enough energy to send the RF signal to a receiver you install with the light and its powered from AC mains.

Link to horrible amazon

If you're in North America, you can source Leviton wall switches. They're UL-listed. They have ZigBee and Z-wave versions. They have plain switch as well as dimmer versions. Once you have radio control over the light fixtures via the switches, you can add battery powered radio buttons that look like light switches wherever you like and make them toggle the Levitons. This assumes Home Assistant availability. If you don't have Home Assistant and you just want to solve this, Lutron has an equivalent solution with their own smart dimmers along with their own remote buttons.