I have one DS with a digital copy installed on it and a cartridge as well. When I want to get something from one copy to another, I just borrow my sister's DS
One copy and one 3ds, I'd spend too much time playing if I had more than one town to take care of. My fiance and brother both play though, so I have easy access to other 3ds's and towns if I need it.
I never thought I would manage 2 towns, but I've found that if you have a purpose for the 2nd town and don't worry about maintaining it, it's pretty easy. By purpose I mean as a cycling town or a hybrid growing town, etc. I don't worry about PWP's or a perfect town rating or expanding the shops or completing the museum in my 2nd town.
One DS XL (expensive enough that I'm not buying another DS, LOL) and two copies of the game. One is a cycling town though I've been focusing on my main mostly. I agree that it takes time to really work and focus on a town, so I know I don't really have the time to have a second 'real' town. Still, it's fun reseting and getting new villagers every time. Though having to go through all the tutorial stuff before you can actually play gets really boring after a while.
3 copies, 2 3ds
Though I lent one of my copies to a friend (I was using it to get villagers that I wanted but now both of my towns have villagers that I dont want to let go of) and I have no intention of getting it back from him.
@__@ I also bought a 4th copy and 3rd 3DS, but it wasn't for myself, it was for a friend who couldnt afford the game/his own 3DS.
ANNNDD... I really, really wanted to get a Japanese 3DS with a Japanese copy of the game, but I don't think that I will. It's too expensive ahahaha OTL;