One second isn't it one company today?... so what's the problem id Jet fulfills orders for Walmart?
One company is owned by the other, but it's still two companies.
Having one company fulfill orders for the other, when dealing with such huge companies, isn't as simple as just "let's do it" and it gets done. That being said, it is doable, and might very well be what they are working on. The logistics and systems need to be worked out.