i have no words of advice to give you.
but i wanted to share my bro and i were very much like your girls. i feel awful about what we made my mom go thru.
we loved and hated each other. one time we literally tore the clothes of each other.
yet if anyone said anything to my bro about me he would tear them apart.
in my case i will say that was a good way to get a 'physical and emotional workout' done. as we wrestled i had to control my anger enough to where i could cause pain but not injury. forcing myself to control my anger was a good thing for me.
I liked the book "Mom, Jason's Breathing on Me..." better than Siblings without Rivalry. One of his basic points is that this is often a ploy for mom/dad's attention and so the less you get sucked into their bickering the better. When someone gets hurt (he says physically, but I say emotionally as well), then you send them both to their 'corners' to cool off. Trying to figure out how the instigator was is an exercise in futility. It takes two to fight, so they both get sent to cool down.
How much one-on-one time does each girl get with you? With dad? My kids get along better (they're 7 and 10, so close in age), when they've had the full attention of at least one parent a couple of times a week. They do even better when they get it nearly every day, but a lot of weeks that's not going to happen. Playful Parenting talks about the benefits of one-on-one time for improving sibling rivalry.
I can't tell you how often I repeat that phrase 'are you the parent?' or "it's not your job" because one of my kids (usually my younger) has said "yeah" or tried to reinforce what I'd just told my other child. There's nothing more infuriating to child to hear "yeah, you really need to" after mom has just told you that you really do need to change your underwear every day!
How do your girls gain energy? Both my kids are introverts. When they come home from school, they separate into different areas of the house. Ds eats his 2 humungous spoonfuls of Nutella (it's the only thing that keeps his pants up!) and goes and plays Wii for 1/2 hour or so. Dd sits down at the computer and composes a story or goes off to her room to construct some sort of elaborate play scenario (or she changes clothes and goes outside in her 'colonial' outfit). Both NEED this down time apart. Maybe you could set up a routine with some time apart?
I wonder too, how much of this is that they're developing different interests and those are harder to manage. It's easier for me because my kids can use the gender difference to 'explain' their increasingly divergent interests. Ds is all about sports, dd is really all about fantasy play. Now at age 8, ds was all about fantasy play too, so I don't think it's as much gendered as one might think. What's your 11 year old into? What's your 8 year old into ? Do they have other kids to play with? That might ease some of the tension too.
If all else fails, teach them how to fight fair. No name calling. No blaming someone else for your problems. (Dd always forgets to save her stories on the computer and then blames ds for shutting down the program without saving. No dear, it's YOUR fault you didn't save. It's not his problem.) Keep to the issue at hand. It only takes about 40 years to learn!
Thanks so much for your thoughtful replies. I did not get back to this thread to reply as I got caught up in crazy work and family schedules. I did read them and started to encourage more time apart, and more time one on one with me apart from each other. I think that me going back to work has been a factor, because their time with me is more limited (I work 12 hour night shifts). I have made it a point to have mommy dates with them at least every couple of weeks, too.
USAmma, I have 11 yr old and 8 yr old girls too. I also liked "Mom, Jason's Breathing On Me". I'm often heard to say "Girls, knock it off," rather than single them out by name.
Good luck with it!