twin behaviour?

by Karyn
(Hamilton, NZ)

One of my 6 month old identical twin babies, has started yelling instead of talking? She isn't unhappy or in pain but is very loud! This has only been going on for about 3 weeks now but prior to that both girls would murmur along to all that would chat to them. Now its hard to hear anything from my other girl. She has also started waking from sleep Screaming at the top of her lungs, seemingly from a bad dream? It takes a bit of soothing then its back to sleep. Unfortunately this means the other twin is wakened and needs to be resettled also.(This can be almost guaranteed at the start of every sleep time.)
Are these 2 things connected and or what are they and how can I reassure her? or help her?