Thanks stringcheese
I have been usng my 4WD's deep cycle battery to charge so far, and the drive home is about 30 mins, so it should be getting enough re-charge from the altenator.
I was under the impression that a 3S1P 1800mAh Li-Po batt would only draw 1.8Amps in a 1 hr charge, so a small deep cycle batt of say 20Amps would provide 8-10 charges - or am I wrong?

If I was going to charge two batteries at once, I would use two chargers.