Maou Evelogia ni Mi wo Sasage yo

Both are isekai, on both the protag (uke) gets reincarneted in a work of fiction, Evelogia in a game and Ateuma in a novel. From episode one we have an established relationship with the counter part (seme), a powerfull guy on the world, with either supernatural powers, status/money who is in love with our main charecter. The two also have 9/8 episodes (around 6min each) they are from the same studio, Hokiboshi  

Both are explicit BL; the characters' power dynamic is similar at first, where the uke is in a caretaking role to the younger seme, and the seme grows more powerful as they get older.  

Both these anime are yaoi isekai fantasies (with characters suddenly transported from our world to a fantasy one) made by Studio Hokiboshi. Both feature fast-paced, very short episodes with interesting character dynamics between the two leads.