achievements for the same game but separate games

What you're looking for is stackable achievements which in this case you can't do.

An older list of stackable games was maintained here https://www.trueachievements.com/forum/viewthread.aspx?tid=492366 however I haven't found
an updated one.

No, the disc games & the digital games are the same games, and so use the same achievement list. The only way you might get a separate list is if it was a game of the year edition or a remastered version of the game.

Some game of the year editions and remasters do have separate achievement lists. I played the first two Metro games on the Xbox 360, so when the Metro games were rereleased on Xbox One, I was able to replay them and collect the new achievements for each
game.

In this case, it sounds like you had the Metro Redux collection which contained the Xbox One versions of the game. The titles you received with your Metro Exodus purchase would have been the same version so they have the same achievement list.

No, there are some games that have different versions for different regions (China, Japan, or Australia for example) or platforms (Xbox One/Windows 10) but there is not a difference between the physical and digital copies of the game; they would have the
same achievement set.

Sorry! Have fun!

No. The only way to get separate achievements on the same game is having a 360 version and a Xbox One version.

HI MerSauta - is some cases games have been released that have a GOTY edition and normal edition with have their own achievement lists, or there's a PC and Xbox version that have their own lists, or there's a NTSC or PAL version that have their own lists
- it's not just about console.