Multiple iPhone Duo reviews covered the device's thermal properties, and reviewer opinions varied. Some found that it was just warm, while others thought it was running too hot at times. Joanna Stern said the back of the device got warm when using FaceTime and taking photos. She had to end a ‌FaceTime‌ call during testing because it got too hot. "Yeah, I'm gonna burn my hand off," she said. " Apple told her the warmth was a sign of the iPhone Duo 's thermal design moving heat away from the chip to keep performance up. "But to me, it just feels like the phone is really warm," she said. The ‌iPhone Duo‌ has the same custom vapor chamber setup and A20 Pro chip as the iPhone 18 Pro , but the vapor chamber is thinner . Apple added a flexible graphite pipe in the hinge to transfer heat between the two halves, giving more surface area for heat dissipation. The Verge said the ‌iPhone Duo‌ heats up when using the camera for a long time, especially if both displays are on for features like Duo Preview. "You can just feel the phone getting significantly hotter when this happens," reads the review. " The site also said it gets hot when running third-party apps, particularly video apps and apps from Google. PetaPixel 's review unit got warm when gaming, photo editing, or doing anything that stressed the processor, with the heat felt on the right side of the unfolded device. Mrwhosetheboss did a 20-minute stress test with the ‌iPhone Duo‌ and ‌iPhone 18 Pro‌ Max.