When the capture files span multiple days then the merged result will be automatically split into files per day basis (24 hours maximum in one merged result capture).
This 24 hour splitting is done two different reasons :
1. To keep the overview when Wireshark span multiple days
2. Generated graphs cover maximum 24 hours to keep the overview
Example of capture files spanning two days :
The logging then shows the split of the result :
And the directory with the result contains the two files (one file per day) :