Oh @sleepingstandingup, I wish that I could offer you a hug. Expressing for twins is HARD. I had two sets and I didn't get past four months with them because it felt absolutely punitive. They were perfectly happy, healthy formula-fed babies and now they're wonderful, bright little things at school and nursery.
However, with my fifth baby I went back to work when she was a fortnight old and I did produce enough for twins (and froze a hefty stash). My routine in the first three months looked like this:
02.30ish: Be woken by hungry baby. Feed baby on one side and express on the other until breast felt empty. If the baby came off of that breast before I had finished expressing, express both. If not, express the second breast once the baby had finished nursing to ensure that both were fully emptied and to encourage greater production.
06.00ish: Baby would doze on the breast, ‘snacking’ occasionally, and I didn’t bother to express.
07.30: Express both breasts before work.
10.30: Express both breasts at desk.
13.00: Express both breasts at lunch.
16.00: Express both breasts at desk.
19.00ish: Express both breasts at desk before leaving for home.
22.30 or so: Feed baby on one side and express on the other until breast felt empty. If the baby came off of that breast before I had finished expressing, express both. If not, express the second breast once the baby had finished nursing to ensure that both were fully emptied and to encourage greater production.
Each time that I expressed breastmilk for my daughter, it was with the aim that I would stay attached to the breastpump for a full thirty minutes in order to ensure that my breasts were fully emptied and to encourage my milk glands to produce more milk at the next feed. Realistically though, there were often times where I would only manage twenty minutes or I would reach a point where I knew that my breasts were mostly empty and struggled to justify the time away from my work.
I just want to say that you're definitely not failing! Breastfeeding is wonderful when it works out, but you have double the babies that most people have and if feeding them breastmilk doesn't work out for you for ANY reason, that's ok. You can still be a perfectly good mother and feed your babies formula and I'm sure that they will thrive on it - mine certainly did.