Hi Dmitry,
You are using the extension for new screens then you will be defining the view extension, so in the view controller you can define new service and use it normally as you do else where. Just define the service and set model to the specific view or UI on which you need new service.
If you need to use both service value in same UI, then proceed the link that you have shared.
If this does not work then make read call to both odata serivce synchronously , then create JSON model and bind it. This should work.
With Regards,
Ramesh Shrestha