Can I caddie for two players at once?

Yes, two or more players may share a caddie. When something you do causes a Rules issue, it's decided like this:

• If you acted on the specific instruction of one of the players, the action counts as taken for that player.
• If no one specifically told you to do it, it counts as taken for the player whose ball was involved.
• If no one directed it and no player's ball was involved, all the players sharing you get the penalty.

The other way round, a player may have only one caddie at a time. They may change caddies during a round, but not temporarily just to get advice from the new caddie. Penalty for having more than one caddie at once: two strokes (loss of hole in match play) for each hole where it happens.

The Committee may have a Local Rule that bans, requires or restricts caddies in a competition.

Source: Rules of Golf (2023), Rule 10.3a.