Is it necessary that I build landing pages for both PPC campaigns and organic search separately. I just recently heard that some PPC networks doesn't allow to have same landing pages.
I don't know that where do you get that information but its not true. Many websites have the same landing pages for organic search users and PPC campaign users. It may occur to somebody that having separate landing pages can be better to track the results but it doesn't matter because you can monitor them separately from your GA account.
Landing pages should be same. Whether its for PPC or organic traffic.
No, use the same landing page for both PPC and SEO.
It would be better if you use same landing pages for your PPC and SEO.