Added a 2nd ssd in optical bay-how do I use it as a separate MacBook?

It sounds like you transferred the drive from another system to this system. If that is the case you’ll need to correct the permissions of the drive by adding your user account to the other drive.

Click your systems primary drive from the desktop (having drives visible on the desktop) go to the Users and your active user folder with it selected. right click and select ‘Get Info’ that will allow you to see the permissions at the bottom under the twisty ‘Sharing & Permissions’ You should see your user account, now click on your second drive you likely won’t see your account listed this is where you need to add it. If you have multiple user folders on the second drive you’ll want to select the folder you want your user to have access and then alter it Vs the full drive.