The rules for the copying spells downtime only permits players that are playing the same adventure together.
Two or more characters playing the same adventure together using this downtime activity can allow one another access to their spellbook; effectively “trading” spells between everyone involved to copy as normal.
The rules only explicitly allow copying spells from players at your table. If there was a process for trading spells between players who are not at the same table, it would specify it or there would have been no reason to specify that the rule as stated was between two players in the same adventure to being with.
Compare this to the rule on trading magic items which says:
[C]haracters can trade permanent magic items received in play with one another on a one-for-one basis.[...]If you’re trading with another character playing in the same adventure as you, the downtime cost is waived. Consumable magic items (scrolls, potions, and magical ammunition) can’t be traded.