Define a function that accepts the parameters of the two lorenzian curves and computes the full curve.
I don't know how many parameters you need cos I'm no mathematician =) Let's say 2?
dualLorentz = @(x, a1, b1, a2, b2) = lorenz(x, a1, b1) + lorenz(x, a2, b2);
Then, define a function to generate that curve, subtract your actual dataset, square the result and sum it.... While you're at it, parameterise the whole thing (ie a vector p of [a1, b1, a2, b2])
objFn = @(p, x, y) sum( (y - dualLorentz(x, p(1), p(2), p(3), p(4))) .^ 2 );
Then chuck it at fsolve or fminsearch - assuming your dataset is in X and Y:
p = fsolve( @(p) objFn(p, X, Y), p0 );