Savannah here from Shopify. :)
You can definitely use multiple dropshipping apps on the same website, but you'll want to make sure that you're factoring that into your shipping prices and that you're setting the proper expectations for your customers.
Let's say that a customer creates one order for an item from Dropified and an item from Oberlo. Each item will have separate shipping costs and times associated, so it would be pretty difficult to create one rate that encompasses both items. You could use an app like Advanced Shipping Rules to create combined shipping rates, or you could offer free shipping for all orders and factor your shipping costs into the price of each product. We've got some information on creating free shipping rates here.
Let me know if you have any further questions!
Savannah | Social Care