nls sslogis: using a dummy variable as an interaction term to compare two sigmoid models

I use the following model in my code:

model <- nls(y ~ SSlogis(x, Asym, xmid, scal), 
   data = data.frame(x, y))

There is a variable (V) with two levels: V1 and V2

• Model 1: Fitting Y to X with a sigmoid function when V=V1 --> asymptote1
• Model 2: Fitting Y to X with a sigmoid function when V=V2 --> asymptote2

Where asymptote1 and asymptote2 are not equal.

How can we show that the asymptotes are significantly different?

Is there any way to use V as an interaction term and get the interaction coefficient? I introduces V-Dummy in the model and it generates the following error:

Error in (attr(object, "initial"))(mCall = mCall, data = data, LHS = LHS) : too few distinct input values to fit a logistic model

If not, what do you suggest as an alternative approach?