After struggling for an hour with instructions in the User Manual and online Support, here's how I was able to pair my Monitor Bluetooth Headphones with a second source:

1) Turn off Bluetooth on the existing/first source.
2) Power off Monitor headphones.
3) Press and hold the Power button for ~10 seconds (in other words, keep holding after the initial "power on" tone) until you hear a second tone.  The LED on the headphone will blink with a sapphire color.
4) Discover the Monitor headphones on the second source and connect.

I love my headphones, but the User Manual and on-line help are deficient on this topic.