Two Geminis together can feel like a conversation that never has to slow down for translation, and that is exactly the problem. There is no anchor. You both light up mid-sentence, finish each other's jokes, change plans three times before dinner and think nothing of it, and for a while that feels like freedom. But mutable plus mutable means nobody is steering. You will plan a weekend trip, get distracted by a better idea on Wednesday, book something else on Thursday, and neither of you will mind, because you are both doing the exact same thing to each other. The trouble comes later, when the relationship needs someone to just decide and follow through, and you both instinctively look around for the one who usually handles that. There is no one. You are staring at your own restlessness in another person's face.
What genuinely works is that neither of you has to perform steadiness you do not feel. Mercury rules you both, so the relationship runs on talk, on wit, on the small thrill of saying the sharp or strange thing out loud and being met instead of managed. You can go from gossip to a half-serious debate about the nature of boredom in the same breath, and it will not feel like whiplash, because you are both fluent in that speed. The comfort of being understood without having to explain your moods, your need for variety, your sudden loss of interest in something you swore you loved last month, is real and rare.
The relationship holds together best when you two treat commitment as its own kind of interesting problem to solve, rather than the thing that kills the fun.