If i remember correctly you can either fit 2 seperate lines or a combined model as follows:
y = frequency
X1 = temperature
X2 = socio-economic class
X3 = Season indicator (0=spring/summer, 1=autumn/winter)
This will give different slopes and intercepts for each value of the indicator variable. I dont remember, but i suspect the two approaches give the same answer unless you can use some process to reduce the number of terms in the combined model.