grub-cloud-amd64 and grub-cloud-arm64 have an undeclared file conflict.
This may result in an unpack error from dpkg.

The file /etc/default/grub is contained in the packages
 * grub-cloud-amd64/0.1.1 as present in trixie|forky|unstable
 * grub-cloud-arm64/0.1.1 as present in trixie|forky|unstable

These packages can be unpacked concurrently, because there is no
relevant Replaces or Conflicts relation. Attempting to unpack these
packages concurrently results in an unpack error from dpkg, because none
of the packages installs a diversion for the affected file.

Kind regards

Helmut Grohne

They also only exist on different architectures.  grub-cloud-amd64 only
exists on amd64, and grub-cloud-arm64 only on arm64.

Okay, Multi-Arch seems to allow concurrent installation.

Bastian