Two letter codenames are apparently hot this year.
Although what NX is hasn’t been revealed aside from being able to play the next Zelda game, a Japanese source appears to have revealed the codename for a handheld system as well.
A statement from an analyst at Morgan Stanley (Japanese link) is indicating that Nintendo has two pieces of hardware in development: the NX (a console) and a portable codenamed “MH”.
No further details about the handheld have been released at this time.
Source: Nintendo World Report Updates