I'm sure that this must come up a lot when writing papers.
Reference A1 gives a lit review of a field. A passage talks about paper B1, and I summerise B1 also. How do you make it clear in your writing that reference B1 was found through reference A1 (after all does A1 not deserve credit for bringing this to your attention??)
And furthermore that if you are paraphrasing A1, who has made reference to B1, you ensure that the paraphrasing is made crystal clear. (For the record lets assume that one has read reference B1 as every good academic should! And that you agree with A1's summary of the work.)