Communicate between multiple ESP's with RS485 Modbus

[admin]

just use RS485 as a serial port in ESPHome. you need to confirm your second ESP relay board's protocol. then you can config ESPHome as this example:

# Example configuration entry
uart:
baud_rate: 9600
tx_pin: D0

switch:
- platform: uart
name: "UART String Output"
data: 'DataToSend'
- platform: uart
name: "UART Bytes Output"
data: [0xDE, 0xAD, 0xBE, 0xEF]
- platform: uart
name: "UART Recurring Output"
data: [0xDE, 0xAD, 0xBE, 0xEF]
send_every: 1s
- platform: uart
name: "UART On/Off"
data:
turn_on: "TurnOn\r\n"
turn_off: "TurnOff\r\n"

here is details about ESPHome UART switch for yaml: https://esphome.io/components/switch/uart.html