Thanks Julian, I'm still having a nightmare with this. We've now tried a trike and my son (Nik) loved it so I am having one priced up, but they are expensive and have alot of extra, hard to find components... Unfortunately everyone who has used one praises them and says its the way to go, whilst two-wheel tandem defenders are in a minority.
I might just stick them both in the trailer for a year then have a tandem delivered to us whilst we're in Mongolia!