but I need to see what is being transmitted to debug.
have you cross connected the lines?The TX of Arduino 1 must connect to the RX of Arduino 2 and the other way around. Also connect grounds so they have a common reference.and the baud rate should be the same of course.
Quotebut I need to see what is being transmitted to debug.Then don't have the two Arduinos tying up the hardware serial port. Have them use SoftwareSerial and two other pins to talk to each other.