Here are the instructions from the Hunter site for operating one fan with two remotes.
"If you want to install one fan operating on multiple remote controls, simply follow the instructions in your Owner’s
Manuals, including the remote control pairing instructions. Each remote must be paired within 3 minutes of restoring
power to your fan."
I've never seen "remote pairing" instructions with any fan remote - of course I've never actually read the instructions. Other than making the setting the same on both remotes and the receiver, is there anything else required to pair? And I don't see how getting power to the fan within 3 minutes would make a difference.