Multiple resources in a single authorization reque

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 use the Graph API, so I added the following: https://login.microsoftonline.com/common/oauth2/authorize?resource=https%3A%2F%2Foutlook.office365.com%2F%26https%3A%2F%2Fgraph.microsoft.com

I've tried different delimiters between the two resources, but couldn't get it to work. Each one resource works separately. I hope that more than 1 resource at a time is supported?

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)

Once the authorization code is available from authorize endpoint, you go to Azure AD token endpoint requesting token for a single resource (using REST call to endpoint or something like ADAL library AcquireToken method depending on your application requirements)
You get back an access token + refresh token as a response to your call to token endpoint. The access token is valid for resource that was mentioned in first call (say graph.microsoft.com)
Then using refresh token you just got, you make another call to token endpoint (REST or ADAL AcquireTokenSilent so that there isn't a popup to ask for user credentials this second time) and get a token for the second resource by specifying the 2nd resource id in case of this call
The access token you get this time is valid for the 2nd resource.
In fact you can continue doing this and hence the name Multi-resource refresh tokens shows up in some places. Although now all refresh tokens are supposed to be multi-resource or valid to be used for requesting any resource that your application has consent for.

Links that can help you in understanding further and implementation