The stories focus on specific adventures the guys have. But as with any group of lives, there are always things happening that don't get recorded. I would imagine they come and go to Engris and Lyrgris with much more regularity than is suggested by the individual stories. You'd think they'd visit Rip Shannon now and then, the lower level, friends all over space and time, and just hang out at the office or in their home. The stories now are mostly logs of their Third Planet Inquiries projects.
So, let's assume that they see Kiernan more often than suggested, and that he and his sister, both, have visited Engris, at the least,