My DD did the screeching thing (and still does at 12 months unfortunatley!) she seemed to go through phases where she would shriek all day for a couple of weeks then 'forget' about it for a few weeks only to 'rediscover' it again (much to my delight obviously! )
LOL janx, let them shriek at each other till they get bored.
She cut two teeth 5 days ago overnight, thought that may have something to do with it, but she seems to be shrieking out of anger/fustration. I've tried the NO and stern looks, but as shes only just 7 months i felt ridiculous and she looked at me with a 'yeah whatever Mum, i dont understand a word you're saying' type look... then shrieked again!