Welcome. The problem you are encountering is the reason for the infamous "gunther saw" (see thread concerning Gunther saw). Basically to make a clean cut into the second baffle you have to cut a standard hole saw in half, then insert a long pipe and weld the toothy end of the saw to one side of the pipe and the other end of the saw to the other end of the pipe. Resulting in a 8 inch(or whatever length pipe you put in) deep hole saw.
There are ways around it, but they are not very pretty. If you use the screwdriver method BE SURE you are punching a hole in the baffle. It is very easy for the screwdriver to slide to the outside and when you hit it with a hammer you punch a whole right through to the outside of the pipe...ask me how I know :-)
