Do you know why twins don't need to keep a journal? Because they can talk with their other half in public, without looking crazy.
Shall I tell you more?
Before paper no one had journals, and it was normal to talk to yourself. So (before paper) if you didn't talk to yourself you clearly had an agenda. Why? The people who did talk to themselves got very worried people were reading their minds because they forgot the difference between thinking and talking.
So often in the fairy tales and stories of the time, twins were evil twins because they were always together plotting and reading people's minds, while their twin minds were impenetrable to all others. Oh yea and to top it off they always had these secret codes, and would burst out laughing together in large groups of people, like they knew something no one else did. These suspicions became so strong that it became genetic.
This is kind of like with cats. By nature cats are friendly to people, but whenever there was a famine, such a friendly edible animal could easily save a family from starvation for a while. So cats though friendly learned to be skittish.
So most people, like cats, are friendly to, yet suspicious of twins, and they're always looking for something to catch them and figure out what those twins are plotting.