Made the changes, restarted weewx and waited for a while but no change.
I just changed the name of one of my extra sensors, ran
sudo wee_reports
then refreshed the page, and the change took.
Maybe double check that the change took in the .conf file (you most likely need root privileges to edit) and that it is the only "Temperature2" and "Humidity2" listed in that section?
I couldn't find anywhere else to change it, so if it doesn't work I don't know. Maybe your installation is a little different, I installed WeeWX five years ago and opted to keep all my configuration files the same with each upgrade.
Maybe look at this section here:
http://weewx.com/docs/customizing.htm#how_to_change_labelsEdit: That above link does show it can be changed in en.conf under the skin you're using, but says it's better to change it under weewx.conf like I think you did.