This sounds pretty normal to me. Normal people grieve. Normal people get angry. Normal people get frustrated. Normal people feel overwhelmed. Normal people have bad days. Normal moms have bad days. And normal 11-month old twins have bad days.
Mental illness, post-partum depression, etc is very, very real and should be taken seriously. However, we've slid into the idea in this culture that *any* emotion other than being happy and calm is pathological. I have a rather big problem with this. If you genuinely feel something is wrong, by all means get help. But please don't feel there's something wrong with you because it's painful to you when your kids have a coordinated, extended meltdown.
Read the book "She's Gonna Blow!: Real Help for Moms Dealing with Anger" It is one of the few books that deals openly with the dark side of mothering.
It is legitimate and healthy to want breaks from being a mom. Find a way to do this. I certainly understand wanting to strangle your MIL (I often want to strangle my mother), but can she provide free childcare? Can you arrange trades with a friend? Start a babysitting co-op? Have a day (or a half-day) when your husband takes the kids and you get some time off?
Be forgiving with yourself. The kids are safe. You made good choices. They screamed for many hours, then went to sleep, which is what they probably needed. You stayed present for 2.5 hours of screaming, then left them in a safe place (their cribs). You removed yourself to protect your sanity and the safety of the children. This is a healthy response, and the one recommended by all the "experts". You did well.
Blessings and good luck. The next 2-3 years are likely to be pretty tough. Toddlers challenge most moms. You've got two.