Usually around 72 hours before departure, the flight is placed under "airport control", and the system will start to indicate blocked seats on expertflyer for those passengers who have tickets but who have not yet selected seats, as well as any spare seats that may be blocked next to status passengers if the flight is not actually full. It may be that your second flight has not yet crossed that deadline, hence no widespread X's shown yet.

What you see today on expertflyer will change a lot between now and departure as people select, change or are allocated seats at check-in (or book last minute, cancel or no show). Both flights could be full (or not), there is usually no way to know for sure from the seat maps until just before departure.