This is currently supported, but you need to be connected to a common WiFi access point. Example connect your iPad and iFly 740 to your home router network or even a hotspot, or common ADS-B WiFi device, not bluetooth.
Then once you build your Flight Plan, touch Flight Plan -> More Options and choose "Share with Local Devices". You should see a popup message to accept that shared Flight Plan on your other iFly devices on that network.
We even tak it a step further by allowing "Always Accept", this will even rubber-band your route as you rubber-band it on the other master device.
I hope this helps. We are in the process of updating our Manual and it will include this.