Every other bank uses identical cards for sole and joint accounts. Starling and Monzo have proved that it’s possible to differentiate in the automated issuing process, but I’m sad they haven’t completely innovated, as I agree with you the differentiation is poor.
At the time, Monzo bleated that it was because Hot Coral was ‘the brand’ but then promptly issued a dog’s dinner of variants for Plus, and business accounts. The unissued metal cards were different again.
Only Monese, to my knowledge, has really innovated in this area in the UK.