You'll can interpolate the two spectra onto a common set of wavenumbers. Then you can add the interpolated spectra together. Here's an example:
% some made-up data
x1 = linspace(650,700,6);
y1 = -0.25*sind(x1);
x2 = linspace(645,705,8);
y2 = 0.2*cosd(x2);
% plot the two curves
figure
plot(x1,y1,'.-b',x2,y2,'.-r')
% interpolate each y on a common set of x values,
% in this case the union of the two x vectors
x = union(x1,x2);
y1i = interp1(x1,y1,x);
y2i = interp1(x2,y2,x);
% add the interpolated values together
y = y1i+y2i;
% plot the interpolated values
hold on
plot(x,y1i,'bo',x,y2i,'ro')
% plot the sum
plot(x,y,'o-k')
% make a legend
legend({'y1','y2','y1i','y2i','y1i+y2i'},'Location','best','NumColumns',5)
% vertical lines showing how the x of each original curve
% maps to an x in the sum curve
xline(x,'--','Color',[0.7 0.7 0.7],'HandleVisibility','off');
