BladesRobinson:
I am surprised you really believe the extra line increases PSD safety.
Take the case that you just brought up about the diver that forgot which way to go. That's only one reason of many in how it increases safety - I've mentioned other reasons already in this thread (ie - when the primary tether can't be freed you still have the SAFETY of both divers having contact with the surface when the backup reaches him)
Having free swimming PSD with the surface having little to no knowledge of where the diver is or what is happening is dangerous IMO.
BladesRobinson:
I try hard to keep an open mind but when I've practiced similar scenarios (without the extra line), no one has ended up "hopelessly tangled" and the drill is faster and, I believe, more efficient. You mention that "all the tangles lead to the tenders," but the way I interpret your scenario, at least one of the tangles leads to the other diver.
I'm not sure you're picturing it right. Bear in mind that good line awareness needs to be in place, as it probably should be anyway. The only way that the divers can get the tethers crossed up is when they're together (or the tenders are doing a poor job of ensuring good tension/angles). Even novice divers can get this sorted out in a couple of dives - all they need is the proper line awareness.
When I say "hopelessly tangled" I'm talking about the first couple of times it is tried.
This method is no slower as the backup is already waiting with his own tether attached, fully geared up.
Some reasons why we like the second tether
- If the back up has to d/c from the primary tether for some reason now we don't really know where he is or what is going on. He could get himself trapped and we can't access the back up now.
- we can communicate with the back up. Without his own tether we can't do this.
- we know exacly where the entanglement is (line distance of 2nd line)
- like I already said, it may be easier to cut the first line to free the diver. Both divers can safely ascend while attached to the 2nd tether