Dumb Question about Paypal IPN Payment Module

I have a question about the PayPal IPN payment module when used on multiple sites.... When setting up the module, you are instructed to enter
http://www.myfirstwebsite.com/ipn_main_handler.php

as your Instant Payment Notification Preferences URL and

http://www.myfirstwebsite.com/index.php?main_page=checkout_process

as the Website Payment Preferences > Automatic Return URL

When setting this up for a second site, you are instructed to
the Instant Payment Notification Preferences URL the
same and enter the new URL of
http://www.mysecondebsite.com/index.php?main_page=checkout_process

as the Website Payment Preferences ? Automatic Return URL...

If the IPN URL is the same for both sites, how will PayPal differentiate
between the sites and process the payment accordingly? Should I
setup a different account for each site? Will this actually work?

I'm sure this has been very well thought out, but I have never
dealt with PayPal and have decided to add it as a payment option
for my customers and really need to know the details on how
this will work.

Any input would be appreciated.

Thanks in advance!

Nelson

1. You basically just have to give PayPal one valid functioning URL in order to check the box to turn the feature on. Zen Cart then overrides that value with the correct store's URL when actually submitting transactions. The setting you enter by hand is used if you submit a transaction to your PayPal account that didn't come from a store where the override URL was supplied with the transaction.
   Related article: https://www.zen-cart.com/tutorials/index.php?article=229

2. It's HIGHLY recommended that you use Express Checkout instead of Website Payments Standard, as it's much more reliable.