Simultaneous fit with parameter constraints

I’m trying to fit two spectra simultaneously with different sets of PDFs, but I have constraints on the ratios of the PDF amplitudes. For example

Dataset 1
PDF1, scaling N1
PDF2, scaling N2

Dataset 2
PDF3, scaling N3
PDF4, scaling N4

N3 = C1 x N1
N4 = C2 x N2

How can I include these constraints in my fit?