Fitting Multiple Curves With Multiple Functions

Here's my problem in principle:

I wish to fit one set of data Energy(x) with the equation a*x+b. At the same time I wish to fit a completely different set of data Charge(x) with the curve a*x^2+b, where a and b are the same in both fits. Is Igor able to fit both of these curves simultaneously by minimizing the summed error from each fit?

Thanks