DM,
Let's see if I can clear this up for you and any others. The Multi-Platform Upgrade is really just that, an upgrade to allow you to use an active data subscription on up to 3 total devices on a mix of platforms (iFly, iPad and Android), without having to buy a complete separate subscription for that additional device.
You do need a current VFR or IFR data subscription to update your 720. You can add the $19.99 Multi-Platform Upgrade to your account. Then you not only can update your iFly 720, but you gain the use of the iFly GPS App for iPad and Android. This gives you the backup option you mention or a secondary device for only $19.99.
At this time, the two or three devices do not communicate with each other or share charts and data. Data sharing and syncing flight plans from device to device via WiFi is on our development list. But you would still need the Multi-Platform Upgrade to use the additional devices.
I hope this helps clear up any confusion. I'll see about tweaking the Multi-Platform description on the site.