For the Arduino you could use a DC-DC converter to about 7.5V and supply that to Vin. Many DC-DC converters have an adjustable output, so you can set them to 7.5V. The Arduino Uno supplies 3.3V (but not a lot of current) for sensors and so.
The Sabertooth motor driver contains already a DC-DC converter with 5V output. It is not recommended to connect that to the 5V pin of the Arduino (a lot discussion on the forum about that), but some say it should be no problem.
I think you can't make a regulated 24V power supply. Just use the battery output to the Sabertooth motor driver. You have to add a fuse for current protection, and perhaps a shutdown relay.
Do you have more specs for the motors ? Is the 15.4A the maximum current ?
Using batteries in series and parallel could be very dangerous. I would not stand next to it. You might try a battery pack for an electrical bicycle. Some of them are 24V 15...20A. They might cost a few hundred euros/dollars, but it is worth it, because of the safety issue.