You can work with as many POD providers at the same time as you'd like. In fact, some of the most successful stores offer an array of products from different providers, as each POD is unique.

To offer products from multiple PODs all you need to do is integrate with them. 

Can customers order products from multiple PODs in the same order?

Yes! Customily will organize everything and send each product on the order to the necessary POD for fulfillment.

Can I offer products from multiple PODs in one listing?

Unfortunately, this is not possible. Each POD works independently so they will need to have their product listing created on Shopify.

What about multiple products from the same POD in one listing?

Unfortunately, this is not possible. At the moment you can work with one POD product per listing. You can offer as many variants of that product as you want, as long as they are variants.
Let's say you want to offer a product as a Poster, and as a Framed Poster, if these are separate products on the POD, you will have to offer them as separate products in your store.