if your still 24v then you will have to conect it to one battery.
In the DC world. amps X hours = AMP/hr. 1 amp for on hour equals 1 amp/hr.
69 amps for 20 minutes is 23 amp hours.
check your battery rating and it should give you the total amp hours available. most are 90 to 100 amp hour or more. thus if your truck was not running, your drain on the battery would be between 1/4 and 1/3 of the battery reserve power.
so yes you could run it on one battery with truck off for 20 minutes and probably still be able to start your truck.
If the truck is running then you should not have anything to worry about unless you had a lot of auxilary things running that would use up your 100 amp alt. output.
Wire size will be your only issue. make sure you size the wire to handle the load. don't have my wire calculator handy but 70 amps at 12 volt will require a decent piece of copper. by the way, you also have to figure wire length when calculating wire size since it is DC. The longer the leads from the motor to the batt. the larger ther wire needs to be. Using to small of a wire will cause low voltage on the other end thus making a short life out of your pump.