I agree with Henrik, that syncing PIC's is the easiest way to proceed.
If you use internal 4MHz OSC in master PIC and set GP4 pin to provide Fosc/4 out, then your system will be in sync. Rest of the PIC's will use master PIC clock as reference.
Two totally independent systems are always out of sync, already from the beginning. Difference might be small at the beginning, but....
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