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.
Are you aware that you can move (swap) the serial port away from the USB port on an ESP8266? If you put:
Serial.swap();
In your code then the hardware serial port moves to GPIO13 Rx and GPIO15 Tx
Personally I would not use serial to communicate between 2 ESP8266s, they are WiFi devices, so use WiFi to send data between them.
So buy some more! D1 Minis are good and cheap.