Wallace can have Monday, Wenesday and Friday, Juan can have Tuesday, Thursday and Saturday. For Sunday they will be allocated numbers for a random draw to deciede who'll be the gym leader for the day, Jaun will have 0, 2, 4, 6, 8, whilst Wallace will have, 1, 3, 5, 7, 9.
Thus problem solved.
(Juan > Wallace for what it's worth.)