Pinescifica (otherwise known as Pinesifica/Pinecifica, and also as Pineswest, a play on "Pinescest" and "Northwest") is the polyamorous shipping of Dipper Pines, Mabel Pines, and Pacifica Northwest. The name as well as the shipping itself was conjured up by yours truly, me, myself and I. In all honesty, it's the perfect solution to whether you ship both Mabifica and Dipifica and are unable to choose between the two. You can thank me later.
For me, my idea of Pinescifica is to not have Pinecest involved in any way, however there are a few fanfiction writers who bypass this idea and decide to include Pinecest anyway, with both twins loving each other sexually/romantically whilst simultaneously loving Pacifica.
Many Pinescifica fanfictions, admittedly including my own, have plots where the twins often irrationally compete against each other for Pacifica's affection. Or a select few have plots where Pacifica is already in a relationship with one twin and decides to compose an affair with the other, without the first twin's knowledge (that is, until later when they're caught or someone's guilt overrides them and confesses.) Majority of these end happily, with both twins deciding that they could share Pacifica in the end and try to make this unusual relationship work out. (After all, they were the ones who instructed Pacifica about sharing in the first place.)