You would first need to create a product for each format of the album file (e.g. one for mp3 and one for FLAC) and upload the respective files for those products (or specify their remote URLs on your server if you're on $18/mo or higher E-junkie plan), then create another product with the "Package files from other products" setting enabled, which would allow you to select the respective individual-format products to "bundle" those files together. Then you would offer purchase buttons for only that final, packaged (bundled) product on your site, so buyers would be issued links for both file formats to download as they choose.