It does make more sense to do than to write about, but it's not very difficult to clone streetpass gifts. You do need to have access to more than one 3DS though!
The streetpass counter is towards the back of the Castele Guild office.
You need to turn streetpass on (and you can only enable streetpass for one character)
Visit your profile, to set your greeting, and an optional gift. The gift will be from your inventory, and can be anything from weapons to tools or ingredients. Gifting via streetpass does not reduce the amount you have on hand!
In this example, I will choose a Sack of Treasure (x1)
Save and exit to the home screen.
Then insert the cartridge into the second 3DS.
When you first boot up, it will act as a new game. Unfortunately (or fortunately, depending on how much you enjoy replaying) you now have to play until you can unlock streetpass in 3DS unit 2.
Set up your streetpass profile in 3DS unit 2. This game does not have any Sacks of Treasure yet, so I will attach a stack of 5xCarrots
Save and Quit, and await streetpass.
After streetpass is registered on both units/games, talk to the streetpass clerk, and check the profiles of your visitors.
You do have to hunt for them around Castele, but streetpass visitors show up as blue spots on the map, so they're not difficult to find. To build up friendship with a visitor, talk to them a lot. It is possible for visitors to be in buildings, tents and workshops. Exit and re-enter the area between chats. You can also check their profile using 'X' to see if they've given away their present.
Eventually, they will think you're SUCH a great friend that you deserve a gift. Ta-da!
So now, Game 1 has 1x SoT and so does Game 2.
Game 2 visits their profile and changes their gift to SoT (1)
Game 1 may change or leave their profile, whichever.
Eventually, the games will streetpass each other again. Friendship levels, thankfully, remain at 100%, so from now on, you only need to find them once you've registered the streetpass and spoken with the clerk.
After the second streetpass, Game 1 will now have 2xSoT. Game 1 visits their profile, and changes to 2xSoT.
Game 2 will have 2xSoT as well, and it will gradually build from there!
The most important thing is, after streetpassing, to find your visitor and relieve them of their gift. They can only be registered with one gift at a time.
