Recharging 2 batteries will depend on how far down they have been drawn and how much amperage your alternator can produce and how long the time period of the recharge. GM diesel powered cars of the late 70's to mid 80's had a second battery and used a 93 amp alternator without any isolator or recharging problems. Of course few Diesel Toronados came equipped with off road lights and a winch, but the point is a properly selected alternator should keep up with the recharge rate as well as the vehicles electrical load at the same time.
