The issue then becomes, if you start a multiplanet takeover thread (listing the planets in the subject line or in the beginning of the rp), you will expect those planets to suddenly be "held" or "exempt from takeover by others" until you complete your thread. But what if the thread is never completed? Or if you have enough words for just two planets in your five-planet takeover thread?
What will happen (or what could happen) is that someone starts four threads of five planet takeovers with a general first post and then presumes that those planets are untouchable by others while they leisurely complete the threads without interference.
So, can it be done? Sure.
But if someone else puts up a takeover thread of their own listing a world you also had listed in a multiplanet takeover thread and
finishes it before you do yours, then they should get the world in my opinion.
That's why it is recommended to stick with on planet per thread since it seems to take a person a few months to finish one thread for one planet let alone one thread for ten planets.