Complex sentence: There was in fact another Dr. Turner in London at this period (a Peter Turner), and a Dr. Turner who had dealings with Henry Percy, 9th Earl of Northumberland in the next decade has been thought possible as either of them.
Split into simpler sentences: There was in fact another Dr. Turner in London at this period (Peter Turner). The Dr. Turner who had dealings with Henry Percy, 9th Earl of Northumberland in the next decade has been thought possible as either of them.