Están ustedes comiendo?
Why are the verbs split in this sentence, but in "Están durmiendo los niños?" they cant be split?
Here is a response I received from a native Spanish speaker and Spanish teacher at SpanishDict.com.
"It seems that the compound "present or imperfect progressive" tenses can be split, but only by a personal pronoun as you indicated with "¿Están/estaban ustedes comiendo?" but it is not natural. It sounds funny. "¿Está/estaban comiendo ustedes?" sounds so much better. With the present or past perfect tenses, you can't do it at all. Either "¿Han comido ustedes?' or "¿Ustedes han comido?" is the only way you can do it. And the first way is the more common or natural."
Based on this and another response, I believe a beginner like me should simply avoid splitting verbs.