You can do that but it's not necessary, your answer is yes the way ford put them in you can hook tender to one battery and both will charge with all battery cables hooked up.

 

By hooking up to one battery. The first will take the charge and only after the hooked up battery is fully charged will the charging start on the later. This is especially true with a battery tender. Because of the low amps being put out. So in this case if you have the tender on one battery and not what the full cycle time to have it fully charged and need the truck. You will have to disconnect the tender and not have charged the second battery. This makes the first battery stronger and the weeker batter will drag down the first battery. Well you know what I mean.

 

I understand, but as far as maintaining a charge it dosn't really matter, now if the batteries are low and you are charging them you have a good point