Not sure about bolting them together as they get bolted into the wall. I would expect you could butt them up against one another when attaching them to the wall to extend length. You would need to measure the distance between the two treads on each side of the joint to ensure uniformity of tread depth.
I did not use masonry nails because they are too difficult to drive them into the cement block foundation. Instead, I used # 10 stainless steel Phillips head screws with matching plastic inserts purchased from Home Depot. First, I placed the stringers against the wall and spotted the holes in the wall with a 1/8 inch diameter carbide tipped masonry drill. Than I drilled holes into the wall using a larger diameter drill to accommodate said plastic inserts. (This allows for easy future removal of the stringer when I need to clean and/or paint the walls or replace the stringers. Another option is to fasten 2 x 10 inch pressure treated boards to the wall and attach the stringer to the board. This works well if your walls are uneven or otherwise compromised.
Only two stringers are required. Try to prevent moisture/condensation from contacting stringers. Reason: Bilco stringers are made from galvanized steel to prevent rust. But I have found rust occurs within a few years and eventually looks unsightly. It would be better if Bilco offered a stainless steel option.
Yes at an added cost
The length of the stringer is 11".
Hello IsFreezing, Thank you for the inquiry about the Size C Stair Galvanized Steel Stringer Kit. This package contains a set of 2 stair stringers. The price is for the package, a set of two. For any other questions, please call us at HomeDepot.com Customer Service 800-430-3376. Thank you for shopping Home Depot.