Hi Perry, thanks for reply.
My experience of using software serial on an ESP8266 is that it doesn't work very well. My understanding is that the cause is that the interrupts that drive the WiFi part of the ESP8266 mess up the serial timing.
Yes, i think WiFi part mess up the serial timing too.
Cita Are you aware that you can move (swap) the serial port away from the USB port on an ESP8266?
And what advantages can I get if I change the port to USB port? The WiFi part doesn't mess up the serial timing?.
Personally I would not use serial to communicate between 2 ESP8266s, they are WiFi devices, so use WiFi to send data between them.
I have to use communication between two ESP and not use WiFi devices, because one of them is used to ESP-NOW connection and other to connect a network, so i can't to use them to communicate both board with each other. This is the reason i am thinking about serial connection to connect them.
So buy some more! D1 Minis are good and cheap.
I already ordered two more ESP8266 
.