15 days means 15 days in a row. You can use the 15 days in 2 months pass. Alternatively, you can check if you can use the 10 days in 2 months pass and buy normal tickets for the 2 days that are the least expensive.

@Paul Guiheneuf 

I follow the suggestion of rvdborgt to buy a 10days in 2 month pass and buy for the 2 cheapeast days a standard ticket.

If you have a Route and you share it with us we can suggest which Route could be cheap ;)