So, it is slightly pertinent to DaveW's question, though his question doesn't hinge on it, but I do want to answer the debate regarding LOS really quick. wtcannonjr is correct that you only have to check LOS when targeting something. Not necessarily when checking range. If Kharhaz will reread the quote from page 17, it says "When you count range to your target you must also check to see if you have clear LOS." I'm not thinking of anything off the top of my head that cares about range and doesn't target something though. Every spell has a target, guaranteed. So this might really be a moot point.
Now, regarding DaveW's question, the important thing here is actually regarding the source of the second attack. The source is still the caster. Unlike Chain Lightning, it is not bouncing from the first target to the second. Instead, like the card says, it is more like a sweeping attack, wherein both attacks come from the caster who is just catching multiple targets in this jet of flame. Be it because the flame is so wide/long, or because the caster pivots and redirects, that's up to your thematic imagination.
But mechanically, you basically are making two attacks coming from the caster, one after the other. A wall between those two targets doesn't matter. However, if there was a wall between the caster and one of the targets, that may stop it because LOS from the caster does matter. Because you have to target them.
I hope that makes sense. I'm a little sicky atm.
