No. It would be 1 sec at f/1.8, if you change to f/9 you will underexpose your film. To answer your question to me: multiply the factors together first, then divide the film speed by that number. For example, if you have two filters, one with a factor of 2, and the other with a factor of 1.6 then:

2 x 1.6 = 3.2

ISO 400 / 3.2 = ISO 125

Since a factor of 2 is one stop, and 1.6 is 2/3 of a stop, this works out to adjusting your exposure 1 2/3 stop total.