Hi.
Sessions are independent, you should set the same time limits in both Joomla and Moodle.
But still this won't make them totally synced: you can keep one session active in one platform and not the other.
In Joomla, Joomdle provides the Joomla session plugin, to refresh the Moodle session when refreshed in Joomla.
I Moodle this is harder to do, and the only way I found back in the day was to use a weird thing:
www.joomdle.com/wiki/Session_sync
I have not included that in the new docs, because I never heard of anyone doing it, and I haven't tested the method in ages.
All that said, I really think it is not worth it to try to keep sessions in sync.
The only way to do it is to make a call to the other system on every page load, thus making the system slower.
Maybe it could be improved, making the call only some times.... but as I said, I never saw any real interest in keeping sessions synced.
