Adding A Digital Purchase

I am wanting to add the option of a digital purchase only after a print purchase has been made. Can it be done and if so how?

You can offer to purchase a download when purchasing a print. 

See: Offering Download Photos With Purchase Of Prints

Another option would be in the product groups of your price lists, "Require a selection from this group before the can select products from other groups".

This option is available when editing a product group (the edit icon next to the product group name). Checking this option will make it so the customer is required to select a product from the product group with the option checked before they can purchase/add products from the other product groups. Example: the customer must select/purchase a collection before purchasing other products. 

I also have an item that Ive placed in a separate group, that I don't want reliant on this rule. I don't want them to be able to purchase a digital if they purchase this product, but I also don't want it locked due to the other group that locks the digital if that makes sense