Sorry, other than the links I posted I can't give hard data. I can give my suppositions but that's it.
My suppositions are that #1) You get some minor damage from the needle puncture itself. The fewer needle punctures the better. #2) I'll "assume" that the meds themselves have the potential to do some damage. So, for everyone the less you use the better. If it takes 12 units to get you boned out then the risk/reward balance indicates 12 units although 6 units would produce less risk of damage (but no boner). However, you don't want to add the extra risk of doing 12 units twice. The same would apply to the guy that needs only 6 units. His risk potential also doubles if he does a second 6 unit dose, although his risk for the meds to do tissue damage are "obviously" lower. #3) There is also the possible risk of priapism due to the residual effect of the first dose added to the second dose, complicating determining proper dosage. That residual effect will (more supposition) be the same for the 6 unit user as the 12 unit user as those are the doses that produce erections for them.
Clear as mud?
R.R.P 2011 Mayo Jacksonville by Dr. Michael Wehle. Nerve sparing - damaged. V.E.D, Viagra and Alprostadil (80mcg/ml) injections @ ~ 12 units. Originally Edex20. Switched to compounded Alprostadil - cost. Injecting 5-6 yrs. It works. Treasure coast of FL.