Will SimpleIndex read multiple barcodes on a page and save the value to the appropriate index field?

Please refer to the Wiki Documentation for the complete Bar Code Recognition reference.

Yes, it can. By default barcodes on any page are read left to right and top to bottom. The first barcode read on the page will be filled into the first barcode field set up in SimpleIndex and so on. You can also narrow down results by matching against a template or dictionary as well as excluding barcodes that are not in a defined zone.