(1) Specify, what do you mean by the distance between curves. If this is the distance between any points of them, a special (though not very much complicated) program is needed; if this is the vertical distance-see the following comments.
(2) If you have the curves as the numeric vectors y1, y2 of the same size presnted at the same common grid x, the maximum vertical distance is just
max(abs(y1-y2))
If the grids differ, address the interp1 function to recast them to the common grid preliminary.
(3) If you only have the curves as pictures, use the digitizing software to turn them into the numeric form. There are a lot of such programs, e.g., DigBit at https://www.mathworks.com/matlabcentral/fileexchange/73785-digbit
