Consider the following example of a coupled oscillator. Let two identical pendulums, each of length $\ell$ and mass $m$ be connected by a spring of force constant $k$. The system has two normal modes with frequencies $\omega_1=\sqrt{g/\ell}$ and $\omega_2=\sqrt{(g/\ell)+(2k/m)}$.
Why is any arbitrary motion of this coupled oscillator writable as a linear combination of the two normal modes? I mean how does the conclusion follow?