I have some kind of connection problem with ATV to iTune after a long period of wait and/or sleep. There is a full thread on this problem here that might look like yours:
In resume, it look like there is a problem in iTune network code that create zombie connection overtime. After a X period iTune stop to respond to external acess even if iTune user interface react normaly. It as been fixed on a previous release and back in a later one...
For me, since im hosting iTune under a Windows XP VMWare server, I created a little programe that restart iTune every hours of none activity. Little drastic but all my ATV get access when I need to. There is a guy (in the thread above) that created a script that pretty much do the same thing if you are hosting itune a OSX operating system.
It could still be nice to see if it is related with those zombie connection.
While problem occur run this command in terminal on both MBP:
netstat -an | grep 3689 | grep EST | wc -l
It will return the number of connections on iTune port. It get bad when you have 99 connections if I remember. Normal behavior should be 10 connections.