Multiple RFID scanner scanning on a form in VB.NET

Chances are you can't, not without significant changes.
The use of a textBox implies it's being entered direct from the RFID reader as keyboard data - and there is your problem: Windows does not store the "source" of keyboard input at all, so there is no way for your program to determine which RFID scanner a particular keystroke came from - and because keyboards are autonomous devices, if two scanners start feeding the keyboard buffer at the same time, the data will be irretrievably mixed together and there is no way - no way at all - to determine which key "typed" came from which actual device.

The only way you will solve this is to talk to the scanner manufacturers and see if there is a way they can transfer the data as a packet instead of as keyboard input. Contact their tech support, and explain your full problem - they may be able to help you, but for certain, we can't!