It is perfectly doable and safe as long as you don't exceed the capacity of your step-down transformer. Nintendo consoles are pretty low power devices so with a 100W rated or higher transformer you should be good to go with plenty of room to spare if you decide to get more consoles.
In fact that's the setup I use on my entertainment center: (Chile 220V~ 50Hz / NTSC-M)
The surge protector on the left goes straight to the wall and runs all the 220V devices (TV, sound system, cable box, etc).
The surge protector on the right goes through a big 220-110 transformer (conveniently hidden behind the cabinet) and it powers my JP/US gaming systems among other 110V-only devices (like my DVD recorder).
I suggest you to get a US power strip to keep it all clearly separated and avoid any possible mistakes, but most likely you won't find one in your local market, so if you buy your step-down transformer online, you should order a US power strip as well.