The following will produce the error "Cannot find a class or type named "Two":
class One
{
Two two;
}

class Two
{
//One one;
}
The following will NOT produce the error:
class One
{
//Two two;
}

class Two
{
One one;
}
The following will NOT produce the error:
void setup(){}
void draw(){}
class One
{
Two two;
}

class Two
{
One one;
}

This behavior is the same whether the classes are in different tabs or not.
Somehow, the preprocessor should detect if there are any additional
classes in a file that is not in "intermediate mode."
I'm using 0156 on XP.

The preprocessor isn't magic, we can only do so much. The second/working
version is perfectly consistent with the way that Processing is documented,
and what 95% of the PDE users will want to do.

If you want complete control over class visibility and setup, add .java to
the tab so that the preprocessor is not used.