Ps4 – If you buy Modern Warfare digitally and insert a physical copy, how can you play the digital one again

Yes, you can play Destiny TTK at the same time, because when you download a digital game onto a different console, it adds a "temp" license for that user, as long as the account that has bought Destiny is on the console the game should be fine.

I hope this helped. Good luck Guardian

I have had this happen multiple times, especially with free PS+ games. I've always had to uninstall the disc version then download the digital version. You can back up your saved games if you're concerned about losing them, but they should be fine.

The disc version probably doesn't install literally everything since it can read some content from the disc, so it makes sense that you'd need to install a new version to play it without the disc.