Just curious about the topology - does controller #1 control the left 20 chars, and #2 the right 20 characters?
Edit: to answer my own question : Info on how to use 40x4 LCD
First 2 lines are controller #1 (which makes more sense). So as a workaround, one may be able to define two instances of any LiquidCrystal library and handle the halves explicitly.