As st2000 stated in the comments, tapping on USB is a complex task and certainly not the way to go.
Maybe there is a program for Raspbian, that pipes the Serial data to 2 programs simultaneously. (I don't know one and that would be off topic here)
So your best way would be to tap the Serial (UART) communication between the USB-Serial chip (which translates USB to real serial UART) and the Atmega2560 (the main chip on the Arduino Mega). Reading Serial (UART) is a completely passive action. You don't disturb the communication with it; only if you try to send something from your tapping device. The Mega uses the first Serial interface for communication over USB, meaning that you need to tap the pin 0 and 1 (marked with TX and RX).
Now about the tapping device: You have multiple possibilities here. You could use the RX pins of 2 other Arduinos or of 2 USB to UART adaptors. But I would suggest to directly buy a logic analyser. That is a small USB device, that let's you plot digital signals (like the serial communication) on your PC. I use one with the software Pulseview. It can also analyse the digital signal for different communication protocols, for example UART. With the logic analyser you would reach your goal of tapping the serial communication, but you would also have a very capable tool for your future Arduino projects (you can analyse all sorts of digial signals with it). They are rather cheap (I got one with little clamps to attach it to a circuit for about 15€).
In the end, it's up to you to decide, which way you want to go and what you want to buy.