Simply interpreted, duplex means something that you have two of. If you write a piece for violin and viola you have, of course, two instruments. Furthermore, when the violinist and the violist are a married couple who have commissioned the work and taken it into their repertoire, the idea of 'duplex' can be to its logical conclusion.
There are no literary or philosophical ideas in this piece, but within the fifteen minute span one may find certain symmetries. Ideas borrowed from other music are played simultaneously at the extreme registers of the instruments and there is a regularity which I personally think provides a viable structure. Consistent countering of ideas between the two instruments can, of course, be seen as a kind of mirror technique and the inverted harmonies create for the listener both predictability and surprise. This simple mirror technique will certainly amuse some but annoy others.
Duplex was composed at the request of the violist Stephan Barratt-Due and his wife Soon-Mi Chung and commissioned by the Rikskonserterne in Norway.