Is it possible to use the same Nintendo Network ID on two 3DS?
No, the official Nintendo site explains this below
http://en-americas-support.nintendo.com/app/answers/detail/a_id/2210/~/can-i-log-into-my-nintendo-network-id-on-a-different-system%3F
The summary is that the NNID are tied to the system. Two system = two NNID.