The OP isn't saying she doesn't like him. She's saying it was/is an abusive relationship. That's two very different things. Moreover, there's a growing realisation in society and relevant professional organisations, of the impact of Domestic Abuse on DCs. Long overdue.
Hi again OP. I'm sorry you've had such a hard time. I wonder if there are any court orders in place, social services involvement or support from Women's Aid for you because of the separation and abuse. You don't need to answer any of that here, but just to say it might give you some leverage in your situation if needed, and I hope you've got some support in real life, and someone to talk to about your concerns.
If you haven't got a SW, can your Health Visitor visit your ex and his partner, to make herself known to them and talk about the DCs' needs? My own advice to them would be to give their bedroom to the girls for overnight stays, and sleep in the lounge on a sofa bed or airbed etc. It's quite easy usually to make a bedroom child friendly and safe, even temporarily. It doesn't sound like you have the sort of relationship where you can suggest this to them?
If they choose to share with them, especially at this young age it is only not seen to be ok if there are significant safeguarding concerns. Given what you've said about your ex being abusive to you, and his partner being negative towards you, and in front of the girls, it's a situation I'd want to keep an eye on as a Mum, or as a professional involved. I also wonder how his current partner and her DS will fare with him in terms of abuse. Perhaps it's not been easy for the 9 year old so far.
All the best OP. I know it's a difficult situation for you, and I'm sure your first concern is for your DC. Concentrate your efforts on giving them a good, stable, loving home, and look after yourself too. 