Chainload grub to grub?

[Michael Peters]

Hi. I'm trying to figure out how to chainload one grub to another.

bios boot, not UEFI.
grub 2.06 built from source.

What GNU/Linux identifies as /dev/sdc has a CentOS 7.9 install with its own
/boot at /dev/sdc1 and the operating system root at /dev/sdc2. I have had
too many bad experiences with a shared /boot between multiple Linux
installs, especially when package managers like to rewrite the grub
configuration file.

What GNU/Linux identifies as /dev/sda has a "Linux From Scratch" install
with its own /boot at /dev/sda1 and the operating system root at /dev/sda2.

Switching the boot order in the bios every time I want to switch operating
systems is a royal pain in the arse, but I also don't want a shared /boot
partition.

What I would like to do is keep the /dev/sda (the drive with Linux From
Scratch) as the boot drive the bios sees, but have a menu entry that will
chain load the CentOS managed grub.

I've tried several things, this is the latest but it also doesn't work
(CentOS entry is the bottom)

====
# Begin /boot/grub/grub.cfg
set default=0
set timeout=5

insmod ext2
# LFS grub root=/dev/sda1
search --set=root --fs-uuid 2322f8e6-1d3d-43ee-becf-67417a46e124

# filesystem root=/dev/sda2
menuentry "GNU/Linux, Linux 6.1.15-lfs-11.3" {
  linux /vmlinuz-6.1.15-lfs-11.3 root=PARTUUID=d92d2b64-02 ro
}
menuentry "GNU/Linux, Linux 6.1.14-lfs-11.3" {
  linux /vmlinuz-6.1.14-lfs-11.3 root=PARTUUID=d92d2b64-02 ro
}

# CentOS grub root=/dev/sdc1 (but can change...)
menuentry "CentOS 7.9 GNU/Linux" {
  insmod chain
  insmod ext2
  search --set=root --fs-uuid 7c405b03-a178-474d-a777-a8ba6359f565
  chainloader +1
}

# End /boot/grub/grub.cfg
====

That attempt based on Windows 7 example I saw.

It results in the following message:

====
error: invalid signature.

Press any key to continue...
====

Is chainloading grub from one physical drive to grub on another physical
drive possible?

How is it done?