This is most likely a limitation of the HP scanning software.
HP scanning applications are normally not configured to connect to two seperate but identical printers at the same time. You can test this by simply unplugging one 1217 and scanning, then swapping cables and scanning to another. If the HP software can successfully scan to each, just not when they are both connected then you will have your answer.
Another good test would be to test scanning from a 3rd party application like the MS Scan and Fax Wizard or Paint. I'd assume these TWAIN based apps will be able to detect the seperate printers just fine.
You might be overthinking your scanning environment. Normally when multiple network scanners are available the Scan to Email or Scan to Folder feature is more appropriate then scanning from the Desktop anyways. Regardless of where you initiate the scan the user will have to go to the physical scanner to complete the job. Might aswell initiate a scan push from the printer control panel as opposed to a scan pull from the users desktop. The job will be waiting in the scan destination when they return to thier desk.
For an official response on the HP Scanning software limitation you would have to contact HP directly.