PayPal and multiple IPNs

I have a single PayPal business account and several websites running Paid Memberships Pro (Plus license). PayPal don’t allow multiple accounts and also, kindly, don’t allow multiple IPNs. What’s best practice in this situation if I want to run the PayPal Express add-on with multiple sites? What happens if a transaction completes on Website A but the IPN on PayPal directs to Website B? Will it just ignore the mid-transaction-site-switch or could it cause problems on both sites?

Thank you!

George