Hello Alan,
welcome to the Polycom Community.
Once the 2nd call is added to your Multipoint HDX call, the Endpoint will re-negotiates video by closing and re-opening the video channel.
You would have to check via a wireshark trace what is happening and if the firewall blocks any ports due to the above.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier
Polycom Global Services