If you're pumping instead of nursing, I would think you'd want to pump in blocks as well. So if you pump every 12 hours, then pump only one breast. That would mean 24 hours without any breast stimulation on one side. But I would think that pumping at the same frequency as baby would nurse, and sticking to block pumping, would be better for decreasing supply than pumping only twice a day & double pumping. Otherwise, you're telling your body that you have two babies who each need milk. And you're increasing hormone release with double stimulation. Most women would not be able to maintain lactation with the schedule you were using, by the way!
Have you heard of storage capacity? We know from research and experience that most women are capable of making enough milk for one, usually two, and sometimes even more babies, regardless of breast size. But, the amount of milk that can be stored between feedings does vary. This is why some mamas and babies can happily nurse every 3-4 hours, with baby content and gaining well, while some nursing couples will need to nurse every hour. If you could go 12 hours without expressing any milk and without feeling like you were going to explode from engorgement, I would guess that you have a VERY large storage capacity. Being able to pump 9 ounces from one breast is another clue (for comparison, when I was breastfeeding twins & even when trying to manage oversupply with a single baby, I could never pump more than 3 ounces from BOTH breasts - small storage capacity, and at that point I'd feel pain from engorgement). For block nursing to be effective, the milk actually has to build up in your breast. There are substances in the milk that inhibit further milk production. The physical pressure of a "full" breast also inhibits production. For you, it sounds like it will take quite a while to get to that "full" point (maybe 24 hours

).
The pump adds a unnatural variable to the nursing relationship. If at all possible, I'd put it away and try to simply nurse your baby. If you're nursing 12 hour blocks, then each time your baby nurses, your breast won't be "emptied", unlike when you pump to empty. Leaving milk in the breast, as your baby will do, also signals your body to slow production. If you do choose or have to pump for whatever reason, I'd try to keep it as much like nursing as possible (pump at the same frequency your baby would nurse, pump only one side & stick to your block schedule, instead of pumping to "empty" only pump the amount of milk you'd expect your baby to drink). If you need to express milk so as not to drown your baby, then try hand expression IMMEDIATELY before feeding & as little as possible - an ounce or less, ideally.
Have you explored ways to make nursing easier for your baby, such as nursing while laying down, "uphill" nursing, nursing MORE frequently instead of less (still using block feedings, though), encouraging baby to come off the breast at the first heavy flow of milk & catching it in a cloth/bottle, nursing while baby is sleepy. . . Some moms even find using a nipple shield can help if baby is struggling with milk flow (be careful, though, because nipple shields can reduce supply

). And some babies handle a fast flow with more ease than others, so perhaps this baby will not struggle like your older children.
If you do decide to try BCP, I'd search now for an IBCLC with experience in this area. Your local LLL could probably help you with this search, or you could simply start making phone calls to local IBCLCs. Not every breastfeeding professional has the same type of experience; ideally, you'd want to work with someone who has had some in this area (not just whoever is closest but who's only worked with moms struggling with low milk supply).