He's describing seeing ghosts.
He's describing how he is seeing ghosts.
He's describing the ghosts he's seeing and how he is seeing them.
Are these sentences equivalent and synonymous? I am wondering if double participles can be used like this.
They're all good (once your correct the typo "hoe" to "how", and they have similar meanings, but they're not synonymous.
In the first one he is describing the experience of seeing ghosts - probably his own experience of it, but not necessarily.
In the second one he is most likely describing the way in which he saw ghosts, eg what he did to make this happen. It could mean the same as the first, but to my ears that would work with "telling", but not with "describing".
The third one is adding further information: the ghosts he saw; and the meaning of "the way in which he saw them" is even more strongly preferred as the meaning of "how".