Using tokens from microsoft graph with sharepoint rest api

You can access parts of the SharePoint API through Microsoft Graph, but I think you're asking about something different - using a single auth flow across services.

If you're using V1 auth (ie. you are explicitly specifying a 'resource' when getting you auth code and token), it's possible but you'll have two unique tokens - one for each resource. This is possible by using multi-resource refresh tokens in which you can fetch a token for resource 2 by using the refresh token of resource 1.