First fimware is ready to be used. 

Was trying to use attiny44 instead of 441 for a week but failed with software UART timings.

Then received 441 which was initially planned and UART worked like a charm.

The firmware is split in two parts: ESP8266 firmware and Attiny441 firware.

For ESP8266 part i've decided to fork Sonoff-Tasmota and use Sonoff DUAL UART protocol for controllers communication. For attiny i've designed firmware by myself. 

Currently the main use-case is covered and I can control channels from RF and MQTT. Still need to address RF binding/cleaning by triggering button from attiny.

Also the project is now available on GitHub: https://github.com/foxel/esp-dual-rf-switch

And finally demo: