You can use an overlay mount
to act on both sets of startup scripts transparently,
with the caveat that services specified on the RW partition
shadow those on the RO one.
Suppose for concreteness in this description
that you are using
that the RO partition is the root partition, and
that the RW partition is mounted at
/rw and contains two directories,
You can do an overlay mount as follows:
mount -t overlay -o lowerdir=/etc/systemd,upperdir=/rw/systemd,workdir=/rw/work overlay /etc/systemd
This can be specified in
# Previous fstab entries to ensure /rw is mounted
overlay /etc/systemd overlay lowerdir=/etc/systemd,upperdir=/rw/systemd,workdir=/rw/work 0 0
fstab entry the two directories will be merged at boot
and will appear to
systemd (and everything else)
as a single set of startup scripts.
Attempts to write to
/etc/systemd will be redirected to the RW partition.
This method isn't specific to
systemd. It generalizes to any init system that acts on the contents of some directory.