I tell my kids they’re my favorite boy/girl all the time, being I only have one each. I’ve never had this issue growing up (thinking my parents favored one of us) so I don’t think in that way. Maybe I should be more sensitive. But truth be told, my favorite child changes from one day to another.
as for pages, I don’t count them and my kids love looking at pages about either one of them. I have a first year book for each of them and then it’s all thrown together in one big family book.