Most version of Linux are not RTOS's (real-time operating systems). So you cannot cause two or more processes to start and run at the same time. But you can get processes to run in reasonable synchrony, like this:
You are quite right to suggest putting the process into the background, but sank1800
hasn't yet defined his interpretation of "exact".
"Exact" with absolute synchronicity: - you'd need two CPUs and some fancy machine-code footwork: Disable all interrupts, wait for the flag.... GO! But even then there will still be be microsecond discrepancies. [ You could search on machine code race conditions
] if you wish to understand more.
"Exact" with approximate synchronicity, you could do a bash script as above, but you might be out by a second or more, depending on the CPU load (which, unless you choose to define it, is going to be unpredictable).
What, exactly is sank1800
trying to start at "exactly
the same time" ?
Then maybe we can help.