Wireshark-users: [Wireshark-users] Time synchronization for capturing packets

I was wondering what would be the best solution for solving following problem.

I'm using two or more separate Windows machines for capturing traffic in a few network points. The problem is that every machine has a different RTC time (even if the difference is a few seconds).
That complicates the correct correlation of traffic dumps.

What would be the best way to solve it?

I was thinking about some external time synchronization between machines. However that would require additional network wiring and a separate NIC to do this.
Also it would require to run some local SNTP software.
My concern also is that it will not allow a precise enough synchronization due to the nature of Windows OS.

As I recall, the timestamp of the pcap packet is given by the WinPcap driver, not the Wireshark itself.

Are there any other, better ways to do it?