Originally Posted by
flying_geek
The only important thing is to use the same to enter and exit. If you use the UK passport to enter, it will get stamped and if you don't have an exit stamp, it looks like you might have overstayed. That can be fixed by showing both passports at some point but might slow you down in the future.
I caused some confusion on an immigration officer's face in Seoul when I produced the incorrect one of my two UK passports on leaving ICN. He was still looking a little perplexed when I apologised in my minimal Korean and produced the passport I entered with. MInd you, now my passport doesn't get physically stamped when entering Korea (well at least it won't when I return), but will be in EU. How times change