Do I need to separate artwork into multiple files if I have multiple products in one order?

It depends on whether you are ordering the same product or different products. For multiples of the same product in one order, you can keep all the designs together. For different products, each one is its own line item in your cart.

• Same product, multiple designs: include all the fronts and backs in a single PDF.
• Different products: add each product to the cart as a separate item, with its own file.

Keeping same-product artwork in one file is fine, but mixing different products into a single file can cause setup errors, so split them by product.