I wouldn't pour one over the other. I would have extra wood so I can make the form smaller if needed and fill in as much as I could and then add on later if I had to.
Worried if I run out and will probably buy more : If I pour concrete into a form, would there be any problems of pouring a second batch over the first if it dried already?
You can't do that, it won't adhere to the dried concrete correctly and will eventually spall off or crack.