Getting Jack or Miranda back to loyal

Like raznagul already pointed out, you can regain their loyalty later.

But you can also resolve the conflict without picking sides if you have enough paragon points. (Maybe renegade points work, too, but I didn't have that at the time of my playthrough.)

It also says so on the Mass Effect wikia:

Shepard can resolve the conflict without picking a side, in which case, both Jack and Miranda agree to put off their fight until after the mission with the Collectors is over.

You can later regain their loyalty.

Source: http://masseffect.wikia.com/wiki/Miranda#Loyalty http://masseffect.wikia.com/wiki/Jack#Loyalty