Dieting with flax seeds and yogurt is it safe?

Please elaborate: I'm not sure what you are asking. Both flax seeds and yogurt are safe but I would not go on a diet that was only those as you would be missing dozens of important nutrients. Choose organic yogurt, best unsweetened and with fresh organic berries. But the bet diet is not a crash diet but long-term changes in diet that you can sustain. I like Dr. Fuhrman's diet- see http://tinyurl.com/ktampgg

These foods alone: Aren't likely to cause problems unless you are lactose intolerant. But if that's all you are eating then you very well could run into problems of macro(think proteins, carbohydrates and fat) and micronutrient deficiency.