Dimming of two rgbw2 with one switch

Yes, you can definitely do that using Shelly's Device to Device communication protocol (see https://shelly.cloud/documents/deve…mmunication.pdf).

Alternatively, you can use MQTT and an MQTT broker (like Mosquitto) to accomplish this. This allows you to be vendor neutral and establish device to device communication between devices from different companies. I have many different Shelly devices (and others); they all run ESPHome with MQTT. I have full flexibility to design any communication between them in any way I want.