The breakdown of Happy Home Acadethe points

The catalog in New Leaf doesn't unlock until you upgrade the Nookling Junction to T&T Mart. This will unlock the catalog machine, which functions in the same way as previous entries in the series did, letting you order (most) items that you've previously collected/sold.

In order to upgrade Nookling Junction to T&T Mart, you must meet the following requirements:

• Complete the 10,000 Bell house down payment
• Spend 12,000 bells at Nookling Junction
• Either 7 or 10 days need to have passed since town creation.

The Animal Crossing Wikia states 7, but Nookipedia states 10. Other sources I can find also state 10, but it'll be difficult to verify whether or not this has changed from the Japanese version until sufficient time has passed from the release date.

You can not have multiple save files of individual towns on the same 3DS system.

Every time you save your game it generates a new, random "Save ID" and stores that ID in both the SD/Savegame and the 3DS's system memory. Trying to load a save with mismatched IDs will result in being forced to wipe the save.

This prevents two things:

1. Having two towns with one copy of the game by using two SD cards.
2. Reverting to an older backed up save to undo changes you aren't happy with.

http://gbatemp.net/threads/question-about-id-checks-with-3ds-and-new-leaf-save-files.349216/

I have personally tested this (having purchased a 3DS and transferring to 3DSXL- my game was cleared since the save data and the system ID did not match). Prior to this, we attempted backing up the SD data, erasing it from the SD and attempted a new save file on the same digital copy. This also resulted in the ID conflict. I have contacted Nintendo on this matter causing an inability to back up data. I am yet to hear a followup reply.

In the case of mismatched IDs, the game will load with a "corrupted data" error. This forces the system to erase any save data associated with the digital download- and any backed up save file will no longer work for AC:NL.