Just got off a flight where I could connect one device for free to the Internet and I went for my iPad.
Is it possible to share the Internet connection from an iPhone to an iPad (and vice versa) with a USB cable? I have an iPad Pro (2019) and iPhone 12 Pro. I have tried connecting but did not see to work, am I missing something or is this not possible?
We used to do things like this about 20 years ago. It was called tethering. But it became virtually obsolete with the advent of Personal Hotspots on iPhone. Is the latter not an option for you?
You can’t use the personal hotspot on the iPhone if it is connected to the internet via wifi. You can tether an iPhone to a MacBook and vice versa using a cable but my question is whether you can use a cable to tether an iPad.
I use the Personal Hotspot on my iPhone regularly but will not work on a plane where you just have wifi.
Yeah, I think for this sort of thing to work you’d have to have used your Mac as the Wi-Fi client. No problem sharing then; all the apparatus is in macOS, for setting up ad-hoc Wi-Fi or Bluetooth PAN, or for representing your iOS device as an Ethernet interface for sharing. I think you’re out of luck using iOS for anything but cellular data sharing (though I’m pretty sure the Wi-Fi could be connected to another network with “Tethering” on, you always share cellular).
I would think these days the only way to get that to work would be to connect both devices via Ethernet, and that means two adapters (well three actually since an iPhone 12’s Lightning cannot TMK go to Ethernet with just a single dongle). But even then, I’m not sure how you’d get it to actually work since IIRC on iOS you lack the network sharing settings that you have on macOS. While any iDevice will make use of Ethernet that’s plugged in, in your case you’d need one of these devices to share its wifi network connection over its attached Ethernet and without such a setting à la macOS, I don’t see how it could be made to do that. 3rd party software perhaps?
Now there’s an idea! If we can have two devices connected via Ethernet, and if we can configure the addresses at either end, it should be possible to run a SOCKS or HTTP proxy on one of them, and configure the other to use that proxy to make requests that will be routed via the default gateway, over Wi-Fi.
Of course, now we’re talking about potentially violating Apple’s and/or your air carrier’s terms of service … but you wouldn’t let a little thing like that stop you using your device to do things that are entirely reasonable and technically feasible, but for stupid restrictions, would you? 
I often hate when people avoid the question and try to talk me out of what I’m trying to do, but please allow me…
For the purpose of the flight, that’s usually just a few hours and you don’t usually have a lot of space to deal with anyway… Any reason not to just use one device or the other? I have a few apps that only run on one device or the other, but that’s shrinking over time, esp. now that my new MacBook can run iPad apps.
But I could get trying to have both me and my wife share the same Internet connection, so if that’s what you have in mind, I get it In that case, I say follow the previous advice to get Ethernet adapters, a cross-over CAT-6 cable, and an Access Point and sell the password to your WiFi service to other passengers for $5 a seat :-)
I wanted access on a 9 hour flight (LHR-ORD) on Friday and I have a flight just short of 15 hours coming up in 10 days time (LHR-SCL).
I do actually use different apps on my iPhone and iPad (e.g. I use WhatsApp on my iPhone and use my iPad for reading and videos) thus I would like both devices connected, it is not a matter of sharing my connection with my wife or other passengers (travelled solo LHR-ORD). Space not an issue but am not going down the ethernet adaptor route. I do have a Beryl AX travel router which might work if I can negotiate the captive portal.
My understanding is that “tethering” (using a USB-lightning cable) allowed a Mac to connect to the internet via an iPhone’s cell phone service. It was not for sharing a wifi connection.
Maybe a decade ago I did try internet services offered on a long flights. I found it was really limited to text services. I suppose it has improved since then but I doubt if it is good for streaming videos.
I load up my iPad with videos and ebooks for long trips. I did find I could easily Airdrop videos from my iPhone or Mac to the iPad but they end up in the Photos app not the Apple TV app.