How to create a swept / chirp signal between a given starting audio signal and a given ending audio signal
As noted by endolith in the first comment, the problem is not well defined, in that, there's no one right way of doing this audio morph/crossfade. 

One method would be to use some form of interpolation between the source and destination spectra as discussed in this paper and implemented in this toolbox. A similar technique is also discussed in this paper.

This paper proposes another possible method inspired by the problem of heat diffusion. Imagine the left edge of a metal plate is held at a "temperature profile" that resembles the magnitude spectrum of the first signal and the right edge is held at a "temperature profile" resembling the magnitude spectrum of the second signal. The 2D "temperature profile" function between the two edges is reconstructed by solving a heat diffusion problem. An intuitively pleasing property of crossfades obtained with this method is that it produces smooth chirps if two sinusoidal frequencies are close to each other, but produces a fade-in/fade-out effect if they are farther apart. See their audio examples here.

I have not succeeded in locating good Matlab implementations of audio morphing algorithms. But there are a couple of good starting points. For instance, the Java library in the Spectral Toolbox has an audio morphing Java source code which can be converted to Matlab with some effort. Another good starting point might be this Github repo.

W. Sethares, J. Bucklew, Kernel techniques for generalized audio crossfades, Cogent Mathematics 2(1), 2015.

W. Sethares et al., Spectral Tools for Dynamic Tonality and Audio Morphing, Computer Music Journal 33(2), 2009.

M. Slaney, et al, Automatic Audio Morphing, Proc. IEEE ICASSP, 1996.