It is important to note that you cannot have power to booth the DC Jack and 36 / 38 SYS_DCIN pins simultaneously. The Dragonboard docs make that clear. So it is much better to have a redundant set of batteries and use a DC source to power the batteries for redundancy than to try and power both the DC Jack & the LS connector. Section 8.2 of the hardware manual for dragonboard says that very clearly. There is not an internal switch to regulate which port the power comes from.
I'm running multiple 3.7V batteries in parallel, with a step up converter to bring the batteries up to 12V. Then I charge the batteries when voltage gets low with a stepdown converter, from 5V to ~4.2V battery protection circuit. This way I can use a 2.4A USB wall-wart to charge the batteries, while the battery bank provides a clean 12V and enough amperage to keep the board alive for quite some time. In theory the board will not draw more than it needs, so having 10A + available in batteries is effective. This way I also draw power for other devices from the battery bank and avoid placing power hungry USB devices on the board itself.