Incompatible Disk

Ok, thanks for the listing. You definitely look like you got it correct. Your APFS formatted internal drive is configured with two containers (partitions). Each holding a different macOS version's sets of volumes. disk3 does look appropriate for macOS Mojave, sans the unique naming of what would be the default Macintosh HD. However, disk2, which you state is running Big Sur, doesn't quite look right, even with your naming convention you used.

For Big Sur, I would have expected five volumes and a single snapshot, as follows:

1. Macintosh HD - Data [yours is George - Data, which I assume is what you renamed it to]
2. Preboot
3. Recovery
4. VM
5. Macintosh HD [This one is blank on yours. I would have expected George here following your naming convention.]
6. com.apple.os.update ... [This one appears to have been renamed on yours.]

Again, I'm not sure what your next steps should be. If it was me, I would start over by completely erasing your system drive. Then install a "clean" copy of macOS Big Sur. Then use a VM manager to run macOS Mojave. However, again, maybe someone else with an idea on how to resolve this will chime in. Good luck!

Sorry, I couldn't have provided you with a direct solution. Maybe someone else here may have some ideas on how to proceed with this situation. With the later releases of macOS, and especially with Big Sur, a lot has changed with the new APFS file system where partitions are now containers. In theory, when you partitioned your iMac's drive, you created a second APFS container. In turn, each container can have multiple volumes. If all went well, Mojave would have been installed on a second container and not in a volume inside the Big Sur container.

The following image may prove helpful in what I just stated above:

To help others, who may be able to help you, providing a screenshot of your current system drive configuration would go a long way. To do so, open the Terminal, and then, enter the following command: diskutil list

The result of doing so should provide you with the breakdown of your system drive. I provided an example of what my 2018 Mac mini system drive, running macOS Big Sur looks like for comparison:

Although a bit late, I would think you would have been better off by using a virtual manager app, like Parallels, to run macOS Mojave in a VM, leaving the system drive for running macOS Big Sur. FWIW, I use Parallels to run VMs of a number of OS's so that I can help answer questions here at the ASC.

I have the same Incompatible Disk notification, have bootable 10.14.6, 10.15.7, & Big Sur Volumes, as well as clones of each connected, I haven't had a problem other than the nuisance notification... I was thinking it might be that APFS doesn't likePartitions in the old sense!?

Mojave doesn't see my Big Sur Install, I can only boot BS by holding Option key at startup to choose BS.

Great news, but I've not found a way other than just ignore it my friend! :)