If the expence is not an issue, pour all at once. You will have to have help. I know you will be happier with the results. Have you ever used a power trowel? If not, you might want to see if you can hirer this out. 20 x 24 is a large pad. If this sets up before your ready, there is no fixing it. You also can't keep a truck on site waiting for you. Let us know how it goes. Ron J,