Synchronize RTC of 2 different MCU's
For distributed system, the common solution is using a GPS disciplined clock. There are many embedded GPS receiver modules from different companies which provide timing information and a 1 PPS signal. A module built specifically for timing usually includes an OCXO for stability. One such module is the Trimble Thunderbolt ($$$). It all depends on the timing precision you require, few milliseconds will be cheap, few microseconds will be expansive. Also, if you have an Ethernet connection, IEEE 1588 (PPTP) is worth a consideration.