I'd agree that they should resurface in a non-threatening way sometime in the future. However, I'd suggest staging an encounter sooner rather than later, as the repercussions of their actions will be more apparent. That, and I think it might be a stretch to find a reason for Tsuto and Lyrie to be hanging around Sandpoint when they are both on the run—I doubt they would want to stick their necks out for the town in that case.
I could imagine them hiding out in the shadow in Magnimar. Perhaps feeding your PCs the information that they escaped and are at large somewhere in the city will be enough to get them to track the fugitives down; if not, adding the incentive of a bounty issued by the High Court of Magnimar might pique their interest and afford you the opportunity to flesh out Ironbriar a bit, either as a foil or faux-friend.
You could then have a fairly interesting side adventure where the PCs navigate the Magnimarian criminal underworld in search of the pair, and once they've cornered them, a group of mercenaries could show up to either claim the bounty for themselves, or to assassinate the PCs at Ironbriar's behest to prevent from unearthing the Skinsaw cult. Whichever motive, it should be clear that the PCs are outgunned and outmatched. Then having Tsuto and Lyrie side with the PCs to even the odds would provide an interesting choice for the PCs to make once the fight is over.
Hope you like the suggestion!
