PoE supply works on both ports with the MegaBoard 1.4
PoE passthrough work on both ports, power comes from one port and goes to the other port. It may even work to both ports when the power enters through the DC barrel.
More information here How the LibreRouter can power another device with POE Passthrough - #10 by Pablo
You should be using revision 1.4.1 and not 1.4.0! 1.4.0 has some issues that were fixed in 1.4.1.
No config needed.