Gordon,
You have two 6V 4AH batteries. If you connect them is series (plus to minus betwween the batteries) you have 12V (2 lots of 6V) and 4AH (not amps, Amphours)
So you will have about half the duration of the 12V 7AH battery.
You two cells in series can (roughly ) supply one amp for 4 hours, wheras the 7AH can supply (roughly) one amp for 7 hours
however your two will weigh about half as much - actually 4/7ths as much.
Now if you run them in parallel (both terminations plus to plus, minus to minus) you will have 6V and 8 AH so you can take (roughly) one amp for 8 hours (but at 6V)
Clear as mud?
andrew