Here you see the input screen.
The area called Nd is the place where you click two times and a box comes up. The box can be used to alter the number of drivers Hornresp calculates. The connections are series or parallel. Changing the number from one to two will tell Hornresp that there is a second driver. You know you have been successful when the ND box shows 2S or 2P.
The s stands for series connection.
The p stands for parallel connection.