
IM YourOnlyOne:
Another possibility is:
They are not cancelling each other's power, rather, Bu Yeon is the one sealing Nak Su's power. Since Bu Yeon only shows up when Nak Su is sleeping, the possible explanation is that whenever Nak Su is sleeping, Bu Yeon does not have to seal her power … she can relax and take over the body.
Nak Su's power is so immense that it is probably taking Bu Yeon all her power to seal Nak Su's power. So whenever they enter the Jin clan home, the power there is increasing the power of Bu Yeon even if Nak Su is in control.
It would also explain why the blue eyes (as of the end of ep17, it appears the blue eyes is Bu Yeon's power, not a soul ejector marker) faded, Bu Yeon has to concentrate on sealing Nak Su's immense power.
To add.
In episode 18 preview, the mages can not use their power. This points to the possibility that the barrier was created by Bu Yeon.
However, since the wild soul shifter is an abomination and doesn't follow the natural order of things, it still has access to magic (possibly from siphoning people's energy and using it as a source) (i.e. necromancy in other fantasy).
Nak Su can use her power inside the barrier probably not because of AoS … the soul ejector was not the reason she is inside Bu Yeon's body -- proof is the lack of the blue marker. Rather because Bu Yeon has to direct all her energy into maintaining the barrier she created. With Nak Su as powerful as Bu Yeon, Nak Su was then left free to use her own power. It could very well be that Bu Yeon is allowing her to.
OR, the ice stone has something to do with the situation as well, which is to Bu Yeon's and Nak Su's advantage.