I am looking for a nonlinear equation capable of approximating a sigmoid curve that can be multiplied by another equation of the same type with different parameters and this product can be made linear in the parameters. I am an ecologist, so I hope I am using the right terms. I originally tried this with the logistic equation.

1/(1*e^(-a)) * 1/(1*e^(-b))

after applying the logit link to this you end up with

ln (e^a + e^b + e^(a+b))

which won't work for my purposes. I need to end up with a linear function of a and b. Any ideas on equations to try out?

seth