IMHO It would be hard. You would have to create some sort of bridge between Dgraph and MongoDB. To synchronize and avoid duplicates.
It is best to migrate once for all. You can export a complete dump in JSON (containing the relations) and if the relations are very complex, You would have to take more steps. This will depend a lot on your context. Database migration is very complicated indeed.
Worse still is when the idea is to create a link/bridge between two DBs with very different concepts. Not impossible. But hard to maintain tho.
If you still want to use both as you mentioned, maybe create a GraphQL server that writes in both DBs, but there are no warranties doing that. You can do it, but it is much better to adopt a solution only.
Unless your application is not copying (broadcasting) between different DB’s. And its microservices are independent. So you could for sure have multiple DB’s (even using GraphQL - which I think would be very good for microservices) in a microservices context. Which isn’t your case, only a possibility.