Why not just buy another $8.50 charger off DX and leave it at work? You could even get 2 more batteries if you feel you might need even extra run time. They would easily be recharged in time for your commute home. Then you could run high both ways and not have to deal with extra batteries and wiring a harness together.
I haven't yet figured out how long my MTE 8-modes last. I did a 75minute ride the other night and had one light on high for +90% of the ride and it still seemed like it was going strong when I got home. The other light was moved from off-medium-high throughout the ride.