So some approaches:
a) Add your own lock, put a phone number on, wait for them to call, negotiate unlocking of both bikes, educate them
b) Battery powered angle grinder - Gonna be noisy, you will attract attention, if the police are called you're gonna have to explain that you are doing criminal damage to someone's lock to get your own bike out
c) biggest bolt croppers you can get, I've had the same lock as that D lock, bolt croppers didn't really even shrug when they went through it (I'd lost the key), slightly bulkier than the angle grinder, but less noisy. You may have issues with people calling police still.
d) These locks have a pretty simple disk detainer lock core, they are eminently pickable with the right tools, if you have the right skills. Track down your local lock sport club and offer them some beer!
e) something I've forgotten
What ever you do, if you go the route of destructive removal of their lock, I'd call the local police and get them to supervise, it avoids having to explain after the fact that you're basically stealing your own bike.
Good luck!
J