Now it is two of them
In the second painting, there are two(!) disciples in sandals. The fashion of the sandals changes between the first and the second paintings. The young disciple's sandals are on the bearded one. One pictures St John letting his older brother, St James (who had the same shoe size) to wear his old sandals for the sad occasion, putting new ones on his own feet... The sandals must be a sign of being a Zebedee. How is that for the interpretation? I do not think you are going to find anything better.