Steffen,
Honestly I think the best solution for you is to rewrite your auto_home.sh and use the "program" map-type of autofs. Like this
/etc/auto.master:
/home program:/usr/local/bin/auto_home.sh
where auto_home.sh looks for the existence of a share /fs1/home/$USER. (autofs passes in key on on the command line to program map-types. see auto.master man page). If it finds said share, it'll return fs1:/home/$KEY, if not fs2:/home/$KEY.
Your auto_home.sh will output either:
/home/$KEY -ro,tcp,soft,intr,nolock,vers=3 fs1:/home/$KEY
or
/home/$KEY -ro,tcp,soft,intr,nolock,vers=3 fs2:/home/$KEY
(Your NFS mount options will vary, of course.)
There are multiple other solutions too (some involving sssd), but they all seem to involve ongoing maintenance when you add / remove users and home directories on your NAS shares. The above solution doesn't require ongoing maintenance.
Spike