It's whatever you feel most comfortable with. But there are advantages to the diode setup, it gives you flexibility.This is the approach I've always taken. It's no big deal to charge both batteries before connecting. Why would you want to use a half-charged one if you're going on a journey that needs two batteries?
You could charge one pack at a time and not have to worry about things. Eg, if you are on a trip, you only have one charger and can spend an hour or so recharging one pack in say a coffee shop. You can take more advantage of opportunistic charging in this way.