Is there line of sight between figs on two different rooftops? The rule for fighting against elevated figures from the ground is pretty clear; is it the same from rooftop to rooftop?

We've been playing that two characters on separate rooftops can attack each other from range if they're both on the edge of the roof, but they cannot if there's one or more squares between either of them or the edge.

Is there an official rule? So far, the default rules seems to be "There is no possible LOS between characters on two different elevated structures.

Yes, you can have LOF between two elevated characters on different rooftops. The rules are:
"If an attacker and its target are both on elevated clear terrain, nothing affects the line of fire except elevated hindering or elevated blocking terrain and other elevated characters."

Edit: Forgot to mention, that quote is from page 16 of the rulebook. Sorry.