U're absolutely right that 2 Roomba running simultaneously in their own "zone" will get job done faster & probably more thorough, since being focus is always good.
Without Home Base, each Roomba will stop & play "job done" tune, I estimate "job done" will happen after 1 hr cos Roomba typically assign 1 hr to do large area.
In that case, u won't even need to upgrade battery since 440 comes with 70 mins blue NiCD battery.
The advantage of Home Base is that battery will not be "starved" for long period after it gets depleted. Prolonged flat battery situation is bad for NiMH battery, but I think should be alright for the 70 mins blue NiCD battery.
iRobot manual says NiCD should be empty during long-term storage.
So a typical dual 440 scenario:
- u disconnect chargers & press Clean for both Roomba
- u leave house
- u come home (say 10 hrs later), both Roomba stopped, probably with Orange or Red power indicator
- u connect both to charge until next morning
- cycle repeats
All in all, I don't see anything wrong with such scenario.
6 mths later u take a peek inside Roomba, and u realise OMG there's so much coiled hair & cakes of dust. Welcome to the 400 series world...

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No need 6 month to empty the dustbin lah , every day b4 plug in the charger just empty it.Whatever vacuum sure got dust one lah, if not that vacuum useless not do work at all.