Is a Yosemite/Mojave Dual boot possible?

Well, apparently there is way to install Mojave under HFS+. I haven't done it myself but if you do a search for "Mojave HFS+" I'm sure you'll find a few links. You'd have to reinstall Mojave of course. Yosemite knows about HFS+.

It is, I believe. But better create volumes from Mojave’s Disk Utility for better Apfs handling.

You can then create an Apfs volume for Mojave and an hfs volume for Yosemite.

Avoid placing Mojave on hfs although possible: any minimim upgrade to Mojave requires it to be on an Apfs volume.

As already written above you’ll be able to read hfs from Mojave but not Apfs from Yosemite.

Akso check your sw: many apps and especially apps that rely on drivers (in my case Softraid) will need to be upgraded.

Ciao

Simon

Yes, it's possible but you'll have to use the Option-Startup command to change startup partitions, only when going from Yosemite to Mojave.

Yosemite can't "see" or understand APFS volumes.

The question was, when booted into Yosemite the Mojave partition was not visible. If the Mojave partition is formatted as APFS, Yosemite won't see it.

You’re welcome.