According to Nicky Romero's Twitter, the two are in the studio together.  We don't know what it means yet, but maybe a collaboration? Maybe they're remixing each other? Maybe it's just a couple of Dutchmen trading studio tricks?

Nicky tweeted:

Romero tweeted again in Dutch:

The tweet roughly translates to "I'm working on the list of 8 melodies that Headhunterz can reenact good." Headhunterz' follow-up tweets suggests a joking and playful camaraderie between the two, suggesting in Dutch that "I'm going to make a list of eight other surnames Nicky Romero could have chosen."