Two Apple accounts on same device

See also Authorization vs. association - Apple Community for some background. Accessing past purchases requires that the device be associated with the Apple ID, which it is then locked to for 90 days before it can be associated with a different Apple ID. If you have an Apple Music subscription then your iCloud Music Library can only contain purchased content from that ID. If you have successfully downloaded tracks from a different ID, e.g. by using another computer, they can be converted to new tracks that can be absorbed into your cloud library. Note Apple Music for Windows is a poor substitute for iTunes. While Apple Music is installed iTunes is limited to managing audiobooks and podcasts. I would recommend removing Apple Music/TV/Devices so iTunes can work normally.

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It's possible to connect to, and use content associated with, two Apple Accounts. Typically what happens is when you try to use content associated with the other account (e.g. install an app, play music tracks, etc.) it asks you to authenticate the second account, but once you've done that you're set.

There are some options to migrate data from one account to the other, allowing the other one to be retired. There are some caveats, though. Apple have a walkthrough here: Migrate purchases from one Apple Account to another Apple Account - Apple Support