2x20min (or 4x10min) is all the pain you need. If it's not hard enough then you too the FTP test too easy and need to input some bigger numbers, I found it was possible to set the FTP 10% higher than the first test, it made subsequent tests/workouts hell, but they were still achievable.
Unless you want to attack really short sections, in which case find them on strava to get an idea of a good time, and search for intervals arround that time e.g if you race enduro, unelss the climbs are really tight, your 30s power (i.e. a short flat sprint mid stage) is probably far more important than your 20min FTP.
Bear in mind that working on one interval will affect the others too, so doing 20min intervals at FTP will lift your 10min, 5min, 1min, 30s, 5s power by incrementaly smaller ammounts, and the reverse is true, hitting the gym and working on your 5s (neuro-muscular) power will do the opposite and slightly improve your FTP. Winter's a good time to work on FTP as it's the slowest to decreace when not training it, so you taper towards events with shorter intervals, peaking with really short ones just before the event which give you the top end, but those gains will be lost within weeks, so no point doing them in the middle of the 'off-season' whereas FTP will stay high regardless.