If you don't want to pay extra for a larger lifepo4 battery (20 amp hour) AND you don't want to pay extra for a second charger, you could just go with a 10 amp hour battery and one charger and carry your charger with you every day. With a 36 volt system, a charger plus a 10Ah battery will be much lighter and smaller than a 20Ah battery with no charger.
If you use your motor only on uphills and pedal without electric assist the rest of the way, 10 amp hours is probably enough for your round trip without charging at work.