The space faring aliens are rodent like, have small tails, are short, around a metre tall, have medium sized brains, love to learn and build, are pacifist, and grew in technology quickly, they can evade quickly, but have fast metabolisms, but their ability to learn quickly, and adapt gives them an edge. They live on the larger of the two habitable Moons.
The medieval civilization aliens are birdlike, a bit taller, learn a bit slower, but have mastered flight, are somewhat pacifist, instead of boats, they have their wings to guide them across the surface, and have a slow metabolism, so they have lots of energy. They live on the smaller of the two habitable Moons.
Life was spread between the planets, and the two are very similar to each other, the rodent species lives on the cool continental world with almost equal parts water and land, and lots of large forests, and the bird species lives on a cool forest world, with much more land then water with most of the land covered in dense conifer trees.