Just so there's no confusion:

The commands /dl and /lp work in both singleplayer and multiplayer.
If you want to download someone's customs (joints, textures, user text, etc.), type /dl [username].
So if I wanted to download your customs, I would type /dl mercykiller.
Wait for it to say "Download Complete" in the chatlines.

Now that you have the user's customs downloaded, type /lp 0 [username] or /lp 1 [username], depending on if you want to load the customs on Tori (0) or Uke (1).
If I wanted to load your customs on Uke, I would type /lp 1 mercykiller.