I'm Dutch and have lived in Amsterdam for two years, currently in Arnhem. Previous posters are right on. As long as you use two different locks and lock the bike to a bridge or something it won't get stolen. Most junks specialize in one type of lock so if there's two they usually won't even try. Also, don't buy those 25,- bikes from junks, when a coppa catches you it's 300,- or something (plus you support the endless circle of biketheft).

Also, Baz. There are plenty of guarded bikeparkings, just a few bucks and your bike is safe.