When you enter a country, your luggage has to pass Customs. In the US, as in most countries outside western Europe, this is done at the first point of entry.

What your post seems to be asking is, can I avoid US Customs? People carrying illegal drugs or other contraband might ask the same question, but the answer is always no.

So it's really independent of whether it's one ticket or two, linked or not linked. But if BA tags your baggage all the way through, you can drop it off after Customs and avoid a full check-in process which might require baggage fees. For that reason, I don't think BA will tag your baggage through.