GET request of more than one resource
The short answer is, you can't. I have an issue posted to hopefully make this, and other filters, a reality soon.

In the mean time, you can however create a custom service resource that uses POST, and from there build a custom callback function to return your desired results.

Alternatively, you can use the Views JSON module (a sub module of Views datasource) or Services Views to easily create a View (with contextual filters, normal filters, sorts, etc) that returns JSON data to you via GET.