my book is written from two completely different characters' POV, and the first page is only one of the characters. So I never know, do I mention this when I send it? Do I send five pages of one and the first five pages of the other?
No. You are very clear in your cover letter that there are two POVs. You send the first ten pages (or whatever number is asked for).
Reading the first ten pages is less about what the book is about and whether aliens arrive, and more about whether you can write well enough to be considered for representation. If I want to find out about aliens, I'll read your synopsis. If I want to find out about all the twists and turns and character development in your book, then I read a partial.
First things first: ten pages to answer--can you write?