A source tells ET that the two are just 'having fun.'
The two shocked fans over the weekend when photos of them packing on the PDA in Italy were released, amid news that Cyrus split from husband Liam Hemsworth, just a week after Carter called it quits with Brody Jenner.
A source tells ET that, prior to their split, Cyrus and Hemsworth actually spent time as a couple with Carter and Jenner.
"The [ex] couples are friends -- the four of them have spent time together in Malibu," the source says. "Brody and Liam are both into surfing and are friends."
A second source tells ET that Carter has always been a fan of Cyrus'; their friendship is nothing new, but the "hooking up" is.
"They've known each other for several months, but the two of them hooking up is very new," the source says, adding that none of their friends believe it will be a long-lasting relationship. "For Miley, she's having fun, and for Kaitlynn, it seems too coincidental that she is hooking up with a huge music star, when that's what she wants most for herself in life -- to be famous."
As for Cyrus and Hemsworth, a third source says the two "love each other and always will," and most of their friends believe "they'll be back together."
"No one thinks Miley and Kaitlynn are in a real relationship, they believe it’s more fun on vacation," the source says. "Liam loves Miley and is hurt, but he knows that she does this."
"Miley and Liam very recently separated. Miley's behavior is no shock to Liam. He is hurt but not surprised," adds the source. "The last several months they've been talking about taking a break. Liam's been frustrated with some of her antics and in turn, Miley claims she 'needs her space.' This seems to be a pattern in their relationship."
The steamy pics of Cyrus and Carter kissing were released just a few hours before Cyrus' rep confirmed to ET that the singer split from Hemsworth. "Liam and Miley have agreed to separate at this time," the rep said in a statement. "Ever-evolving, changing as partners and individuals, they have decided this is what’s best while they both focus on themselves and careers. They still remain dedicated parents to all of their animals they share while lovingly taking this time apart. Please respect their process and privacy."
According to an eyewitness, Cyrus and Carter were "not trying to hide [their PDA] at all" while lounging poolside at the Il Sereno hotel.
"There were other people sitting by the pool," the eyewitness recalls. "They were fully on a date, getting drinks, just the two of them. They even got a couple of massages at the hotel."
Hear more (and see the pics!) in the video below.
Reporting by Adriane Schwartz.