Do you have the grid values of the chi2 for the two chosen parameters keeping the other fixed ?
If this is the case you can use methods of the TH2 or TGraph2D painter classs for finding the contours.
No it is not obvious, you need to decide exactlly what you want.
You have chi2(x1,…xN)
and you can have:
chi2(x1,x2) keeping all the other parameter fixed. But for which values ? Their values for which chi2 has a minimum ?
a profile of chi2(x1,x2) , which means for each x1 and x2 you find the minimum of the chi2(x3,…xn). This is the algorithm used in Minuit/Minos for finding the contours
Marginalize the chi2 distribution by computing the Integral(chi2(x1,…,xn)dx3,…dXn)
Then for computing the chi2 function from the data points you can use a machine learning regression algorithms