Secretly give one of the two volunteers the spike that has a neodymium magnet attached to the flat end, and tell that volunteer to 'hide' the magnet from the audience, by holding the spike with the magnet in a closed fist.

When the volunteers have a 'contest', there is a clear winner. Make a 'big deal' about the 'loser' not being able to lift any nails. (The winner must have a 'magnetic personality'.) Then, let them trade spikes and allow the audience to see that it was a 'trick'.