I wanted to write to 2 SD cards simultaniously with one Arduino Mega. Used all the relevant pins including CS on 53 for the first SD card and 13 for the second SD card. Can be any digital pin I think but this one makes the LED blink too). Before the SD.begin(13) of the second SD card I do a digitalWrite(53,HIGH). That de-selects the first SD card. and ...
Well had a bit of luck after spending all day on this.
Arduino UNO (sender): Like mentioned by chrisl, use Serial.write and not Serial.print
UNO Wifi (receiver):
So, instead of mySerial.read I used mySerial.readString.
This is most likely a obvious mistake to almost everybody here; ...
You can't use the LED on pin 13 on Uno, Nano or Nano Every, if you use SPI . It is the clock pin of the SPI.
The init function of the display initializes SPI which dedicates the pin to SPI peripheral and disconnects it from pin IO peripheral.