Assuming you have an XY array of data, dividing it in two part can be done by filtering the data against a chosen criteria:
XYdata = [linspace(1,10,8)',rand(8,1)]; % increasing X and random Y data
x_threshold = 5;
%filter the rows in column 1 that are more than x_threshold
XYdata(XYdata(:,1)>x_threshold,:) ;
%filter the rows in column 1 that are less than x_threshold
XYdata(XYdata(:,1)<x_threshold,:) ;
Then you can do your calculation on those filtered array.
