If they were both connected to Internet, you wouldn't have to do anything as the second BBB would update its time via NTP as soon as it's online.
If they're not connected to Internet, it should be a matter of running an NTP server on the BBB with battery packed RTC. The other one can run an NTP client. First tutorial I came upon:
I don't know how the NTP server will behave if there's no Internet in the first BBB. Configuration may require some tweaks to account for this.