The unseen side of bodysharing

Years after freeing the monsters from the underground, Frisk and Chara have gotten used to sharing a body together. However, after a long day or work and a movie night that went off tracks due to an unwanted complication, the two of them discover one more way that they can use Frisk's body to have some fun. Chara and Frisk are adults & afam and amab respectively. This is a lemon !