Severely obese, will i lose more weight if i only eat half my calorie allowance like 1000 and still work on my treadmill 20 mins a day?

Weight loss: Correct!!! Weight loss is about creating a calorie defecit (burn more than you take in). the average human can burn 1600-2000 cal /day a bit more with exercise (20min on treadmill about 150 cal) . If you take in 1000 cal the deficit is 600-1000 cal. The problem is living on 1000 cal/ day is almost impossible.

Need both: If your appropriate calorie intake is 2000 calories/day, reducing this by 500 calories/day will result in weight loss of a pound/week (3500 calories=one pound) Cutting more calories will result in more weight loss. BUT, cutting too low, & 1000 calories per day is near the too low point, may lower your metabolic rate and curtail weight loss. Burning calories through exercise will also be necessary