Good question, @Gareth. I don’t think the Card Member field comes through in our feeds.
If the cards had different numbers there’d be many ways to skin this 
… but with a single card number you’re probably going to have to tag the transactions manually (… though, if you’re good at coding it might be possible to reconcile the CSV with your Transactions sheet with a custom script).
Consider having a look at tags and the Tags Report template. Though you’d need to apply the tags manually, this might be a good way to track expenses by cardholder without disrupting your categorization. If you tend to use different vendors than your partner, you could even make your tag assignments using Autocat 
.
Good luck,
Randy