First of all, thanks for sharing. Sharing an experience and asking for a critique takes courage.
Most of the obvious issues have been addressed. I do want to discuss two potential issues that have not been covered..
Why with 500 lbs in your tank, were you using your buddys air?
You were not out of air, just low on air. So keep your buddy close, but use your own air. Once you had reached zero, I could see it. But at this point, all you are doing is causing your buddy to go to zero twice as fast, and then what? You were not out of air, but were responding to a low air situation as if your were; as a result, with both hands busy and unable to monitor your assent and you may have came up too fast. This action of buddy breathing when it was not necessary at this point caused the problem.
Now lets talk about the first uncontrolled assent. Looking again at your buddys profile, it shows he took about 3 minutes to ascend from 70 feet to the surface. That assent rate is not really all that bad, until you get to 35 feet. In fact it is well within the 30 feet per minute rule.
I do see the alarm for too fast an assent at 35 feet. From 35 feet, it is hard to tell how fast your assent was from that point, but at your computer you can better gauge it. I may even argue, that in hind sight, your ascent rate was not really too fast over all. Since you could not see your console, what was your buddy doing, besides going up too fast? Did any of your buddies make any attempt to tell you to slow down? Why did all of you come up too fast? Then the issue is mostly likely air in the BC expanding.
I would also suggest you take another look at the profile of the dive and at your dive tables. The NDL for a dive to 120 is 12 minutes. The dive tables dive time is, as I understand it, computed as go down in a controlled manner, bottom time and then surface in a controlled manner. Your dive exceeded your dive tables.
This dive was not well planned or executed from a NDL dive time point of view, or air management either. This dive indicates that while you have experience (50 dives), you may have become lax and are not executing your training well.
Now about your practice of "regurlary breathing off your buddy's octo becasue they use less air". That is just plain unwise. The dive is called based on the first person to run low on air, and call the dive in time to reach the surface on that persons tank. ALWAYS! To fail to do so, is to ride on the edge of dangerous. Also, why are you planning and executing deco dives with only a NDL certificate and training? Also, never let yourself be talked into exceeding your experience and training based upon someone else has more experience. In a situation, it is you not them you will have to rely upon.
Bottom line, is that you had a "situation" and your reponse indicates that one word we men never like to admit to "panic". Buddy breatihng with 500 lbs in your tank - panic; ascending too fast - panic; realizing it and dong an uncontrolled decent to over 100 feet with less that 500 lbs in your tank; and who knows how little left in your buddies tank - panic. Panic is often the result of a lack of training, practice or expereiance or any combination of those items. Revisit the basics, practice them in a controlled envrionment, and enjoy the diving.
So, count your lumps, learn from the experience, and buy a lottery ticket, because if your were not injured your were lucky.
