Well, this is a tough one. Personally, I haven't seen any Penobscot "double" bows that are dated before 1900. That doesn't mean the design is new but it sure looks that way.
There are bows in The American Museum of Natural History that are labeled "Penobscot" that are the double bow type, recurved, and simple D-bows. This tells me that there is either confusion on the subject or that the tribe used many types of bows. This doesn't help to answer the question, though.
The surviving Penobscot "double" bows were probably made after 1900 but the design might not be new. I think the old design, if it existed, was more similar to a cable-backed bow than a double bow. In other words, there would be more cable and less wood. The smaller bow on the back would be there only to elevate the cable off the back of the bow.
There are some advantages to an elevated cable:
You don't have to put wraps on the bow in order to keep the cable in place.
The bow can be tillered with the cable in place.
The cable can be replaced more easily.
The cable can be made in two sections, which is easier to make if we are talking about using sinew.
The cable tension can be adjusted.
This last point has been proclaimed by some to be a unique feature of Penobscot bows. It's not unique. Some Eskimo bow styles had adjustable cables on the backs.
Modern bow makers tend to focus on the bow on the back and make it quite long in relation to the primary bow. This is a modern design (after 1900). In my opinion, the smaller the second bow, the more authentic the design, if it existed at all.
