Welcome! Most likely it has to do with the relationship of a Customer to Order being One to Many in your model. A customer will have multiple orders, so that relationship will be a one to many relationship. In your DataView (a single object widget) set to your customer, you would need to drop in a Reference Selector, for example, to see your Orders. Or better still, drop a Template grid or List view inside of the DataView and set the Source to your Order over an Association to start.
Thanks Nolan, you put me on the right track but I don't understand your comment about dropping in a reference selector in my DataView to see my orders. Here is what worked for me
Customer_Overview page
Customer_NewEdit page
Order_Overview page
In Order_NewEdit page