You can pass two independent variables to your function but you have to combine them into one array. Assuming Re and Pr are both column vectors, the array for them becomes:
RePr = [Re Pr];
your function becomes:
function [ Nu ] = fitfun_Nu(param,RePr)
Nu=param(1).*(RePr(:,1).^param(2)).*(RePr(:,2).^param(3));
end
and the call to nlinfit becomes:
param=nlinfit(RePr,Nu,@fitfun_Nu,param0);
That should work.
