How do I fit two different equations?

A way to fit two different equations is to combine them in a single curve fit definition using the if-else operator. If you had two functions such that:

y = F1(x) for x < x1 y = F2(x) for x >= x1,  you could enter the following type of definition in the Define dialog: ((x < x1) ? F1(x) : 0) + ((x >= x1) ? F2(x) : 0)