Two sites competition function to describe the competition of a ligand for two types of receptors.
Names: A1, A2, log(x0_1), log(x0_2), f
Meanings: A1 = top asymptote, A2 = bottom asymptote, log(x0_1) = first center, log(x0_2) = second center, f = fraction
Lower Bounds: none
Upper Bounds: none
The concentration at half response: