This does not sound right... but it can be confusing how Cura lists the OctoPrint connections.
The thing to understand is that the items in the list for the Connect to OctoPrint dialog are not your printers. They are connection configurations to OctoPrint. You can create as many of them as you like. They are not part of your printers profile in Cura.. they are a separate entity.
I had some issues with this when I was first setting things up for multiple printers.
This is going to sound like I am telling you something you already know... but follow along, from the sounds of your issue, you don't quite get how the printers relate to the connections.
When you go into manage printers, you should see both your printer listed. One is "active" the other is just listed.
When you select the one that is not active, it does not become active. You have to take action and tell cura that you want to make it active. The easiest thing to do is just select the printer you want to work with before you try to manage printers in cura, that way it will be the active printer when you open that dialog.
If you now select the "Connect OctoPrint" button, it will bring up a new dialog. If your printer is already properly configured and connected to an OctoPrint instance, you will see the Connection settings info on the right hand side of the dialog. If not, you will just see a list of previously created connection configurations. Hard to say what you see since you have been working on this for a while.
If you are starting fresh and only have configured your first printers connection, you should see that connection in the list. (you likely named it the same as your printer.) Its not your printer though.. its the connection info you are looking at now. Not the printer.
To set up the second connection for your new printer, select the Add button. Then fill in the info Name it something like the name of your printer but add a suffix to it. This will help you understand the difference between the printer profile and the OctoPrint Connection configuration. Once you hit OK, you will land back on the connection listing dialog. Likely either with no connection selected or the one you created for your first printer. Look closely here. It almost never does what one would expect.
Select your new connection.. It should be the last in the list and have that unique name with the suffix.
The interface here does allow you to have it ask OctoPrint for access and the API key but I find it easier to just log into OctoPrint and create a user named like "E5Cura", then create an API key for that user with the appKey of Cura.. Creating a username like this will keep you from makes in connections between OctoPrint Instance.
Then you can use the API key in the dialog in cura. If you paste in the API key and click to the side(outside the text box), Cura will fetch a few bits of info like the URL and Username showing you that it can talk to your cura instance. Make any additional selections of options you would like and then click "Connect"
Not a lot changes here. The button will change to "Disconnect"
If the url and username line up... you should be good.
Don't click the Disconnect button... just close the Connect OctoPrint dialog using the [X] at the top right of the dialog.
Then close the printers dialog using the [X] at the top right of the Preferences dialog.
Now pull in a Model, slice it and you should see the "Print with OctoPrint" button we all love.
I hope this helps you or anyone with similar issues. Good luck.