Fitting 2 data sets simultaneously with two different functions that have some shared parameters

I have a global fit that I am trying to do. Where I have two data sets. dataset A and dataset B. Both A and B are vectors of size 85X1. I am trying to fit them using exponential fits. The two fit expressions are as follows:

fitA = a1*exp(-t/b1) + a2*exp(b2) + c

fitB = 1-(a1*exp(-t/b1) + a2*exp(b2) + c)

Is there a way to do such a global fit with the fit() function or lsqcurvefit()?

Thanks!

Joel

P.S. I have matlab v. 2013b