So it may need work if I wanted to use 4x40, probably the largest standard character LCD around? The 4x40 has 1 extra pin, an extra enable to control the second H44780.
"I think if you treat the two controllers as two separate LCD interfaces it should work fairly easily."
This will work, but why don't you just use a library that is written for the 40x4 displays? Start here --> Google Code Archive - Long-term storage for Google Code Project Hosting. and follow the links to the current version which is was LiquidCrystal1.0 the last time I looked.
Don