This code should help. It assumes you want to count all the area between the two lines as positive. The more crossings there are, the more this code will give an overestimation.
cP6(k,:)=P6;
cP7(k,:)=P7;
x1=cP6(:,1);
y1=cP6(:,2);
%x2=cP7(:,1);
y2=cP7(:,2);
%if x1 and x2 are not the same, you will need to resample them,
%or use polyarea instead of trapz
A=trapz(x1,abs(y1-y2));