At one point, the O/P clearly said he wants to issue his command to both of the two Phytec boards from his PC. If that's all he's going to do from the PC, which I doubt, then all he has to do is to send the command, in parallel, via the serial port, to both slaves. There's no reason two boards can't receive serial input from a single source, and a PC serial port can certainly drive two receivers. Unless the cables differ widely in length, which, of course, he can control, they should receive the same signals at the same time and respond accordingly. They must, of course, be in exactly the same state at all times.

The only way to ensure that the two slave boards are in precisely the same state is to control the clock inputs to both MCU's such that they're exaclty in phase and exactly at the same frequency, and to control exactly, the rising and falling edges of RESET to each MCU. Once they're in synchronization, given exactly the same inputs, they should remain in exactly the same state. At that point, if they receive the same "start" (or whatever) command from the PC, via the UART, they should produce the same interrupt response, if that's how they process serial input. If their oscillators differ by one part per quadrillion, all bets are off.

I think we've lost Walter, however. He's probably concluded he's barking up the wrong tree.

RE