As you do not state exactly what you have in terms of hardware we will assume okay.
So, if you have two standard Arduino UNO's each with a single hardware asynchronous communications port and connect them together using TIA-485 drivers, why on Earth are you then doing serial.prints to the computer, assuming your computer is connected to the Arduino via USB ?
You only have 'one' serial port, so use it in either TIA-485 mode or via USB but not both.
People so often fail to see this point and wonder why their code does not function.
Paul - VK7KPA