If you are in a situation where you "need" distance and separation, I don't think a duplex will be enough distance. Anxiety, frustration, stress, guilt, anger...all of that happens AFTER the "move" and then being on the "other side of the wall" in a duplex will not be enough separation for you.
My husband had suggested that he would live in the basement and not come upstairs and that would have NEVER worked. It does not provide enough privacy for you. You would not be able to focus on your kids or yourself because your spouse would always still be there, just around the corner and most likely within hearing distance. The kids will be confused. Every time they disagree with a decision that you make...they will run to your spouse for comfort or help to change your mind.
You will lose all authority/decision making power..because your spouse will always be five seconds away to go to for a different answer or decision.
My husband moved about a mile and a half away...and many times I thought that was too close because my kids would just get on their bike and go over to his house when I told them they had to do chores or homework. He would not back me up and send them back home. He played the role of "savior" from their bossy mom.
If you are going to be the primary parent and responsible for getting them to do chores and homework, then he will end up being the "nice/good" parent. If you plan on separating responsibilities evenly between the two households, then the kids will resent doing chores in two places, but it will help with the homework situation.
I think the most important thing is for the both of you to talk to the kids together and agree on how things will work so they know you both are on the same page and they can't play one of you against the other. That is a HUGE thing that I heard over and over again in my support group meetings.
My husband explained the whole "situation/arrangement" to my three kids (4 to 11 years old) when I was gone one day and I have no idea what he told them, but when I came home, I ended up being the "bad guy" from that day forward in the eyes of my kids.
It's going to be hard for you and the kids no matter what. If I could do anything differently over the past fifteen years, I would have spent more one-on-one time with each child separately and set aside a few hours a week where I would do things just for myself (relaxation, fun, time with friends). You need to take care of yourself too if you want to be a good mom!
It's a sad state of affairs, but these situations are not unique anymore. Kids don't want a separation to happen in their family, but half of their friends most likely are in the same situation. Kids are resilient and will bounce back. They just need to know that you love them and will always be there for them. Both my husband and I worked really hard at not changing their school and social lives. We both still showed up at all their soccer games and performances and sat next to or at least near each other. They need to have that from you.