Thank you for your inquiry.
I want to create two different sites using PayPal Express Checkout with s2Member Pro Forms, is it possible to use one PayPal account with my 2 websites? Do i have to use different PayPal accounts for each of the websites?
It is not necessary to have multiple PayPal accounts to use s2Member + PayPal.
From the docs
(Dashboard -› s2Member® -› PayPal® Options -› PayPal® IPN Integration -› More Information):
*Quick Tip* In addition to the default IPN settings inside your PayPal® account, the IPN URL is also set on a per-transaction basis by the special PayPal® Button Code that s2Member provides you with. In other words, if you have multiple sites operating on one PayPal® account, that’s OK. s2Member dynamically sets the IPN URL for each transaction. The result is that the IPN URL configured from within your PayPal® account, becomes the default, which is then overwritten on a per-transaction basis. In fact, PayPal® recently updated their system to support IPN URL preservation. One PayPal® account can handle multiple sites, all using different IPN URLs.