While a single 40-foot container will work out cheaper, 2 x 20 footers give you more versatility in terms of what you store. You could store one type of product in one 20ft container and something completely different in the other – which may simplify loading and storage.
Two 20-feet containers also mean you have two access points – one set of doors for each container. This means that your door will never be more than 20ft from the furthest corner of your container, which isn’t the case in a 40-footer – unless you choose a side door or double door option.