This describes how to handle multiple consent dialogs while using the Goa library as described in Oauth2 for Apps Script in a few lines of code (which you should read first for background).
The library, cGoa, is available under this project key.
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The problem

As discussed in Goa Setup, when a consent dialog is initiated, a new session of your Apps Script is initiated – meaning that the context and flow is lost. Consider the example where authentication to multiple services are required in the same script, some of which might need consent dialogs and some of which might not.
Take this extreme case, where authentication is needed for all these services, how is going to be possible to keep track of which have been done and which have not?
Since Goa is able to detect whether consent dialogs are needed, and also to detect which service is being called back for each initiation, it is possible to handle a mixture of consent requests in the same app.

Pattern for multiple Oauth2 consent requests.

The pattern actually turns out to be quite simple. The app will be re-entered for consent as many times as there are consent dialogs needed. Whether the app instance is because of a consent dialog request is detectable by the presence of a goa request name, in which case we return to get consent. If not then we continue to search the list of packages for those that still don’t have a token. Eventually all the consent requests are completed and we can fall through to execute the app.
For more like this, see OAuth2 for Apps Script in a few lines of code.

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