How do I grant a second user complete access to a user's Home folder? I want a second user (but NOT all users) to have complete read/write access to a user's Home folder – both users should have the same level of access on all files within, even those created by each other.
This appears to do the trick:

cd /Users/UserTwo
sudo chmod -R +a "user:UserTwo allow list,add_file,search,delete,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" ./
sudo chmod -R +a "user:UserOne allow list,add_file,search,delete,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" ./


As long as you apply these specific ACL entries for both users, everything seems to work properly. You don't want to give other permissions (like writesecurity) because those are best kept as administrative functions.