mid 2010 Mac mini with 2 internal disks, the one with 10.6 doesn't see the other disk (10.12) in startup disk in system preferences

It does occur to me that if you see the disks in Disk Utility, but not in the Startup Disk Preference Pane, it likely means that the OS can only recognize compatible OS (same version or earlier, and probably compatible Bootcamp installations) as being bootable.

In that case, what you want isn't possible because Snow Leopard can't recognize Sierra as a bootable volume, whereas Sierra can so recognize Snow Leopard.

Try using rEFInd which is a graphical boot menu which should see both bootable operating systems. rEFInd will place itself onto the hidden ESP (aka EFI) partition on one of the disks. Nothing actually runs within the OS except when you run the install script which copies the rEFInd bootloader & app to the hidden ESP/EFI partition (it will reside in a folder called "refind").

I would suggest clearing the PRAM and seeing which drive the system boots from by default when no default Startup Disk is selected, then install rEFInd to that physical drive so if the PRAM is later cleared, then it is more likely that rEFInd will still boot automatically (not guaranteed, but odds may be in your favor).

File Vault? Snow Leopard doesn't know CoreStorage either, and only encrypts the user's home folder, not the disk.

What file system on the Sierra disk?

If you converted the Sierra disk to APFS (which I believe was an option for Sierra and enforced in High Sierra) - then no, the Sierra disk will never show up in Snow Leopard. Snow Leopard doesn't have APFS drivers (which only appeared in Sierra) and can't recognize the disk as being a readable device.

What do you see in Disk Utility (from either/both OS)?

What about on the command line:

diskutil list

I assume oldmini is your Sierra install and oldmini-too is the Snow Leopard one?

My previous comment on Snow Leopard not being able to recognize a later OS is probably right, then.