We currently use the following authorize url: https://login.microsoftonline.com/common/oauth2/authorize?resource=https%3A%2F%2Foutlook.office365.com
We want to also
I think what you're trying to do here by passing multiple values to resource
parameter directly will not work (probably not a supported scenario, but I'll wait till someone from Microsoft confirms or I find Azure AD documentation stating exactly that. In the meanwhile, here's an old blog post that says something like this, but it's a blog talking about SSO and old from 2014 :), so don't want to rely solely on this.)
Below I'm explaining how you can make this scenario work by reusing refresh tokens and without passing both resource ids in same call. (NOTE: This approach will work for Authorization Code Grant Flow but not for Implicit grant flow like a JavaScript based SPA, because no refresh token is returned in that case)
Links that can help you in understanding further and implementation
Call Multiple Services With One Login Prompt Using ADAL
Refresh Tokens for Multiple Resources
This SO Post.. look at comments as well.
This SO Post