Your Shopify store can have multiple suppliers in case you sell a variety of products that a single supplier is not able to fulfill. For example, you can set Printhouse to print your art and also have a second supplier to produce other type of product.
To do this you need to set the "location" (that's how Shopify calls the suppliers) for each product, that way your store will know to which supplier give the order request.
Here are 2 articles from Shopify where you can learn more about Locations and how to set them up on your store:
If you have any question please let us know.