If you are currently controlling a unit that can hire an NPC, you can left-click on the NPC stack, then go to the tab for unit actions and a hire button should be available.
I have found in this instance that if there are 2 NPCs, the hire button reflects the cost of only one NPC in the tooltip, but you need to have enough Gildar to hire both for the button to not be grayed out. So it's a bit of a guessing game to know how much Gildar you actually need to recruit them both.
That's my recollection, anyway.
My best guess is that there's some algorithm for how NPCs move, and in certain situations they end up getting on the same tile and then they just keep together in the flow of the algorithm and never split again. That's only a guess, though.
