If would be helpful if we could rely on a piece of information from the mongod logs to synchronize the information in a common timeline.
Sometimes the logs have:

• no timezone information
• clock drift between hosts

which makes it difficult.

One possibility would be to use a unique token or an information from a heartbeat and have the 2 hosts log this info in there corresponding logs. Searching for the string, in both logs, would give you a synchronization point.
For example:

• host A records a time and a unique ID
• host A sends the info to host B (may already be available through the heartbeat)
• host A logs the info (the recorded time would be different than the log time, however we care about the first one)
• host B logs the info received, with its own timestamp
• host A or B would add the perceived latency it knows about the other host.

This token should only be needed once per couple of hosts at a frequency of once per hour, or once per day, or once per log file.