Just tested this, the answer is yes.
The numbers on the right do the following.
1st = The alignment of the object, this is the number you want, having one planet at a 180 alignment and the other at 360 gives you two objects on opposite sides.
2nd = The size of the object your adding.
3rd = The distance from their orbiting object, keep this the same if you want them to orbit at the same distance.
4th = The speed of the object, important to keep the same if you don't want your two planets crashing into each other (not that anything will happen but it just looks weird.
For example, this creates two planets who are of equal size, distance and speed but orbit the same object at opposite ends:
SectorEntityToken corvusIII = system.addPlanet(star, "Corvus III", "gas_giant", 200, 300, 7500, 400);
SectorEntityToken corvusIIIA = system.addPlanet(corvusIII, "Corvus IIIA", "barren", 360, 120, 800, 20);
SectorEntityToken corvusIIIB = system.addPlanet(corvusIII, "Corvus IIIB", "cryovolcanic", 180, 120, 800, 20);
It's also possible to stack planets on top of one another without a problem. I don't know which planet will be shown in the game but the game doesn't make a fuss about it and they do all exist at the same time.