Near-resonant coupled oscillators mix their motion into two equal hybrid modes

Evidence

The closer together the two individual test frequencies are, the more the pendula are nearly resonant with each other, causing them to interact more strongly. When the pendula are of equal length, both new pure modes are equal mixtures of red and blue motion, but they differ in frequency and the relative phase of the two oscillators; this was the case just discussed in figure 10. — Eric J. Heller, Why You Hear What You Hear- An…, 1035