The alternative way to interpret this question is 'how do I have one web server + URL handle authentication from Banno's Authentication Framework e.g. for a sidebar navigation menu item and a plugin card?'.
In that case, it would be possible but you would need to have two External Applications defined.
Recall that an External Application can have one, and only one, defined link type. As an example, plugin cards use the PluginCard link type. Creating an External Application with the PluginCard link type makes that External Application eligible to be configured as a plugin card for the Dashboard.
It sounds like you are hoping to use something like the DebtRelief (Loan assistance) link type, since that falls under the category of "Main navigation links". Unfortunately, there isn't any to have an External Application with two link types, nor is there a defined way to 'redirect from this link to that link' when it comes to plugin cards.
A way that you might handle the 'how do I have one web server + URL handle authentication from Banno's Authentication Framework e.g.
for a sidebar navigation menu item and a plugin card?' question:
Assume that your institution has its own web application server that can handle authentication requests at a Redirect URI, for example https://myawesomeinstitution.com/developerstuff.
In this case, you could set up an External Application with link type PluginCard and Redirect URI of https://myawesomeinstitution.com/developerstuff that you can use for the plugin card. You could set up a separate External Application with link type DebtRelief (as an example) with that same Redirect URI of https://myawesomeinstitution.com/developerstuff. This would give you the UX that you want, i.e. a sidebar navigation menu item and a plugin card, while keeping development costs pretty low since it all uses the same Authentication Framework.
If you wanted to somehow differentiate the UI presented to the user (e.g. if the UI you want for the plugin card wouldn't make sense as a launching point for when the user clicks the sidebar navigation menu item), then you could adjust the above by setting up slightly different paths such as https://myawesomeinstitution.com/developerstuff/sidebarthing for the sidebar navigation menu item and https://myawesomeinstitution.com/developerstuff/pluginotherthing for the plugin card. That would use the same web application server, just handling different routes so they can serve up different content.
As an aside, it's entirely possible that your web server may not be selecting the correct credentials (Client ID + Client Secret) if there is only a single URL being used (e.g. https://myawesomeinstitution.com/developerstuff from the example above). That would certainly explain why Banno Mobile seemingly requires an 'unnecessary' login since there is a mismatch between the credentials expected for the sidebar navigation item versus the plugin card (or vice versa). Using two different URLs (handled by the same server) would make it easier to ensure that the correct credentials are being served in the right context (e.g. the hypothetical examples from above of https://myawesomeinstitution.com/developerstuff/sidebarthing and https://myawesomeinstitution.com/developerstuff/pluginotherthing).
