If you want to sync your portage tree and your layman-overlays with one command, you can use
All you have to do is to create
and place a :::bash *
If you want to have a cronjob for that create for example a file named /etc/cron.weekly/eix.cron
and put something like
You can then delete all other cronjobs like emerge --sync and layman -S
As usual man eix has all the info you need.