Oh dear!
You need to put being kind and thoughtful on the agenda.
Find out what the favourite rewards are for each dd. Everything they do must be listed up, so favouite tv programmes, cooking with mum, day trip with dad etc etc. The best treats are worth say, 100 reward points (day out with dad) and saying something nice or doing something nice for the other worth 5 reward points etc which they can build up.
Tell them fun things only happen to kind and thoughtful girls.
Tell dd1 if dd2 does kind things to her they will both get a reward. Tell dd2 if dd1 does kind things then they both get a reward.
Get them working up to the weekend by 'banking' rewards, so if they are able to play together for half an hour (say) without shouting then they both get a reward.
If dd1 can say something nice about dd2 then they both get a reward and so on. Basically the idea is to turn them into competively trying to be nice to bank rewards in order to get to watch tv/have a nice snack/go out with dad etc etc which is why everything they enjoy has to have a reward value.
You extend this by saying who can tidy up their bedroom the quickest? If you help each other then you get bonus points/reward.
Even if they are quiet in each others company you can go into the room and say, wow! you both are getting on really well so you both get reward points.
If they shout or are mean then they lose 1 reward point and so on.
This worked so well for my dds as they soon got to enjoy the praise from the reward system that they knew they wouldn't be going out for a nice weekend trip etc unless they got on.
It is important to say to them I am going to give you 2 minutes to sort out an argument or you both lose a reward, or you gain 10 reward points if you sort it out yourselves, thus giving them an incentive to sort it quickly.
If you give them 'treats' without insisting on kind/thoughtful behaviour first then it won't work.
Good luck!