Is it correct that you can only see across hill hexes if they are attached to each other. In other words, If I have a unit on a lone hill hex, he cannot see a unit on another lone hill hex that is 2 hexes away.
If the two hexes between the hill hexes are clear you can see the unit on the hill.
Use the standard line of sight rules the hex you or your opponents units are on dont effect line of sight only whats between them.