Force new created file permission in folder

you set an ACL on your user (u:), but not on other users or a group or the world. chmod is only valid for the files you created. For new files the permissions are set by umask. This uses the inverse of the flags, so if you want new files to have 775 (equals to u+rwx,g+rwx,o+rx) you use 002 as umask.

I would change the group to a common group you both belong to, in Ubuntu that would be a group you would have to create as usually the users get their own groups.

addgroup users
usermod -a -G users yourusername
usermod -a -G users otherusername
chgrp -R users /var/www

And change the default umask to include groups. In /etc/pam.d/common-session set the pam_umask.so to

"session optional pam_umask.so umask=0007"

Users would have to login to a new session again and umask should show the new umask

groups
id

both can show you which groups you belong to.