
Idk why but im waiting for some animal facts in the comment section to explain what could be the inspiration for this

Ask and you shall receive.
I'm unsure for what exactly u r looking for but animal facts are all what I can offer.
So basically I'll start with the assumption that the mermaids here are more of fish than mammals just based solely of the gills and tails.
In some fish species being intersex occurs naturally meaning both female and male parts. Some are not naturally occuring tho but can happen do to exposure of carcinogens and just waste water.
But those that occur naturally like in here the male would have some ovaries in him meaning can reproduce with another male.
While some female fish wouldn't need the help of make fish to be able to reproduce they can become intersexed but not naturally occuring except in really small tiny fractions usually there is something called sex reversal which is used in fish farms but that isn't the topic now.
Now to the octopi for now I do not think there has been any record of octopi going out of there way to fuck a fish but they're known for punching fish or yeeting then out of the way.
Can they be intersex like fish are ? Nope they cannot nor can there be sex reversals either.
In conclusion can there be mpreg ? Hell yeah.
Will it scientifically work? I'm unsure of how closely related those 2 species are and if they or if they not can interbreed and have non sterile offspring but for the sake of manhwa magic yes the can most definitely have babies and I hope we see the horrors they create.
Oooooh angst~