You can (or could) download the same season ticket to two different devices at the same time. The turnstiles should block the ticket being used more than once. I had to do this when the system was first introduced as my mobile (at the time) didn’t always work for NFC and the barcode option wouldn’t scan using the slot (because the barcode wouldn’t reach the scanner no matter which way I turned it!), so I would then whip out an older smaller (non NFC) mobile and scan that. Must have looked real dodgy to onlookers. Ended up just getting a new phone instead.
Purely by chance I have one of my seasoncards downloaded on two phones after the end of last season (I didn’t go to the Villa game and normally hold two on my phone). Technically my copy shouldn’t update as the second copy was downloaded later, but I’m waiting to see what happens. They’re both showing 1st Aug 00:00 at the moment.
All that said, definitely ticket transfer is the best way when sharing with another I would say if you trust each other to do the transfer properly.