Its called a crossover. There's this big crossover thing going on where Brave, HTTYD, ROTG, Tangled are all mashed together. There are various ships (you know, fan pairings) that include Jack x Rapunzel/Hiccup/Merida, Hiccup x Merida/Rapunzel, Rapunzel x Merida, etc,. This pairing is, I believe, one of the most popular ships in the crossover so that's why you've probably seen them together a lot. Try researching on ROTBTD/Rise of the Brave Tangled Dragons, you might learn some more about other crossovers from there.