I am astonished! When I read your op i assumed you meant your dd would have to change secndary schooly after 1 year, which is totally different. Blimey, I really don't think it's any biggie, a little kid changing at the start of primary schools.
To disrupt a chile near the end of secondary edication is imo totally unfair. It could have life changing concequesnces. And that imo would apply here even if the kid didn't have to change school, but suddenly had dramatically reduced contact with their dad at a crucial time.
I would strongly advise you to forget all notion of moving ahead without your dh. If you are unhappy and finding it tough where you are, I can guarantee you will be in a for a huge (unpleasant) shock if you suddenly find yourself having to cope without the daily support of your dh.
My dh moved with work somewhere I didn't want to go. I was very happy in my home and didn't want to disrupt the kids. I thought him working away M-F and coming home at the w/e would be the perfect solution. I'm quite strong and independant and do probably 95% of the parenting and running the home anyway so I didn't think it would make much difference. I was very wrong and in for a huge shock. It was hard, very very hard. We survived 6 months, then all of us moved together.
Your dh has agreed to move. He has said 2 years.A house move can take easily 6 months or more. I'd recommend you stop looking at the situation negatively and seek to work through the time constructively. eg months 1 - 6 house hunting/getting a feel for the market, doing House doctory improvements to your current home to increase it's marketabilty, checking out primaries in your new location balh blah, so when you do come to be ready to move, it can all go smoother.
Oh and if you're so unhappy, consider visiting GP for help, etc.
I think your dh in this case is right, and I would say you moving without him would be a huge mistake. Not only because you will find it much harder than you thought, but your dh will take it as a real attack. Thinking logocally, how would it work. If you are away, would he only see you at weekends? But then what would he do about seeing his children? That would be preventing them seeing each other wouldn't it? So you're stopping him seing his dd (and wife) during the week, and stopping him seeing his other children at the weekend. YOu are putting therefore your dh and your marriage and your dd in an impossible situation, for the sake of a few months.