shiro85:
Either option leaves you at 90ft with minimal air supply.
For the OW application, the immediate remedy is to ascend if your buddy's not immediately present. If you can get the air turned off en route, that's gravy.
Lets look at this. First of all, I don't care to do 90 ft dives with a single and no redundancy. A 90 ft ESA thrills me not in the least so my solution is to calmly signal my buddy, shut down the offending post and switch to my backup and end the dive in the most appropriate way from there. Works every bit as well in OW as it does in a cave.
With your immediate ascent with one buddy not immediatly present, you not only have a diver facing a 90 ft ascent with little or nothing to breath but you have the divers seperated on top of that. But going further what if it's a 90 ft dive on a wreck with a current and you're down current from a moored dive boat. Now an immediate ascent may mean drifting to Cuba before the boat can come looking for you. How good are you at deploying a surface marker during an ESA? I'd rather switch to another breathing source and make my way back to the boat or at least have the luxury of a slow ascent deploying a marker on the way up.
Now if buddies are working together you have him for a backup breathing source. The problem there is that most divers aren't taught to plan their gas so they each have enough left to get both back and the typical 80 cuft tank results in a pretty short dive if you do plan your gas that way for a square profile 100 ft dive. So, even with your buddy there, you're liable to end up sucking on a dry tank. If you do have enough gas to get back to the boat and up, that trip along the wreck navigating the current sharing air on that short little hose is going to be a real chore.
I threw in the wreck and the current but recreational divers are doing dives like this all the time without a plan, equipment or skills that give them a good chance of getting back if things go wrong. In fact, I remember a dive on the Papoose. 90 ft to the top of the wreck, 120 or so to the sand and a boat load of rec divers. the current was strong because the weather had been bad so you had to stay behind the wrck (below 90 ft). My wife and I hit the sand (yes in those days we hit the sand), poked around a bit and decided we didn't have enough gas to be there and headed up. We hung around near the line watching the jacks and a big cuda and sure enough a few minutes later the other started bee-linning it up the line with near empty tanks, some having to skip any kind of safety stop. If one of them had an equipment failure on that wreck it would have been a real mess because no one had enough gas to help any one. Could you imagine any of those divers doing a free ascent in that current and OOA? BTW, two of the divers who came rushing up, near OOA, seperated because one was in more of a hurry than the other and skipping safety stops were an instructor and a DM diving together (sort of). The next dive was a max of about 65 ft with no current and there were no near death experiences. What a difference.
Of course being able to reach ones valve might not have been enough to help. here wasn't a single diver on that trip that had the equipment or skills to have any business doing that dive including my wife and I. That trip was a real hint that we needed to start doing something different. Well, more like everything different.