The first inserted Sim Card will lock Galaxy S7
The leaked presentation from Telcos shows that Galaxy S7 / S7 Edge will get carrier lock from the first inserted Sim Card.
If you are using Sim-Free or Carrier Unlocked, then you have nothing to worry about. This concerns the Carrier Locked phones. If you have this kind of phone, then the first Sim Card that is inserted will lock the Galaxy S7 & S7 Edge