I think I have found the solution to the splitting of curd while making Dahi Bhindi. It wasn't difficult really because that's the way the Gujarati Kadhi is made. Instead of adding gram flour (besan) to the oil, you have to mix it with curd (just make sure there are no lumps) and then add to the dish. It will stop the curd from curdling or splitting and give the dish a creamy texture :-)