as nath said... i fitted a 16in fan to my calais about 6 weeks ago, my old fan was locked solid and could not even turn it by hand in the end...
depends on ya cooler piping but if u have a standard inlet and ur cooler piping runs in front of the radiator twin fans is a no-go...
with the single 16in and only used the top half of the fan shroud and i also used the davies craig thermo control switch with it, takes a bit of muckin about to get the fan to cut in when u want it too but all up, took me 2 hrs from start to finish(fitting fan, wiring controller and setting the controller up)
hope this helps mate, any Q's send me a pm..
Larry