The Merman's Children And Operation Chaos

The premise of Poul Anderson's The Merman's Children is that magic used to happen but not any longer whereas the premise of his Operation Chaos is that magic happens now but in a parallel universe. Both premises recognize that magic does not overtly happen here and now. I have just noticed a structural and thematic parallel:

The Merman's Children, about merpeople, is divided into -

Prologue
Book One: Kraken
Book Two: Selkie
Book Three: Tupilak
Book Four: Vilja
Epilogue

Operation Chaos, narrated by a werewolf, collects -

"Operation Afreet"
"Operation Salamander"
"Operation Incubus"
"Operation Changeling"

Thus, these two volumes present ten kinds of mythical being. The Broken Sword also features a changeling.

The merpeople kill a kraken and are helped to return home by a selkie. I have not yet reread Books Three and Four. I have been noting nautical terms:

"...the halfworld lived yet along this littoral..." (The Merman's Children, p. 95)

"Vanimen tried to work clear of the lee shore..." (p. 96)

"Pole out the starboard clew!" (p. 97)