First, it definitely isn’t related to the port. Every device on a network could use the same port and it wouldn’t matter. What is does mean is that you need to sort out the IP Addresses on your network and make sure your router and the two receivers are properly setup either with Static IPs or DHCP Reservations:
Thanks for the faq. I static-ed both receivers in addresses outside of my DHCP block. I had to restart the iphone but both receivers were detected and things are working again!