One question to come out of this is related to laurin's: Given that you want a given amount of bottom time, should you break it up into one dive or two? Ignoring logistical considerations (which is somewhat silly, since something like very heavy seas can offer risks greater than deco risks), would you rather do two 20 minute 170ft dives or a single 40 minute 170ft dive?
Kinda depends on the dive itself - if I understood your question correctly, you're asking "if you want a given amount of bottom time at a specific location for a specific task" then the 'easy' answer would be 1 big dive, especially if you include the descent to BT (we usually do, for conservatism). Incidentally, it's the same stuff that needs to be figured out with the gas plan - all usable, 1/2s, 1/3s or less, just the other way around... The longer it takes to descend and/or get to that location the more it favors one long BT, cave diving being the extreme example.
On our dives the effective bottom time for 20min on a small wreck at 170ft would look like: 5min (or more) descending and getting to the spot, then maybe 12-13 minutes of effective bottom time (shooting or whatever) and 2-3 min swim/scoot back to shot (we don't usually drift deco). So for two dives you'd have about 24-26 minutes of BT for any task-oriented stuff, compared to the 32-33 min for the longer dive.
I'd say for 2 shorter BTs you need to be more efficient, but have the luxury of a SI in between where to adjust the plan for conditions.
For the shorter BTs having a scout team to check out the shot location and move it or place a spool/reel is essential, you really don't want to waste any time figuring out if you actually hit the wreck or not. =)
The above is obviously fairly location specific, somewhere else you might have a narrow time window dictated by tides, or the visibility might not necessitate too much scouting. Or the daylight period of the day might prevent multiple dives - up north during the summer we have the advantage of long hours of sunlight, so we can spread out the diving.
This is a bit theoretical for me, since OC gas limits have been favoring 2 shorter BTs this far for me. I have to admit I've been eying RBs lately, though I'm not sure if I'd be ready to extend the BTs very much on RB. The main advantage would be topside logistics, which can be a nightmare for a big OC team doing 2 dives a day for a week or more... Essentially hitting a long good weather window is no good for us, because we'll have to spend a full day blending after two days of diving anyway.
