The dual filter adapter I have seen on the market is in series. In my humble opinion the most important part is FLOW. When I bought my cobra it had the dual remote filter, and was plumbed with 10AN lines. The motor subsequently lost all
oil pressure as the small lines and dual filter proved to be too restrictive. After I had my engine rebuilt I went with 12AN lines and a high flow remote filter and cooler to accomodate this size line. I haven't heard anyone recommend dual filters, especially if you are using a high performance filter.
my 2 cents