They also kind of have warped senses, and have only ever said they do it because they enjoy it, so there doesn't seem to be any emotional element to it. Which is why I find it tough to define them as dating.
Akiho and Fuyumi pretty much jumped straight from "childhood friends" to "functionally married" without passing "Go" bothering with the various intermediate steps. To all appearances by now their relationship is such a natural and self-evident part of their lives they don't even think about it and would probably get rather confused if you asked them for some kind of definition.
Hell, Akiho keeps wanting to kiss other girls.
She's also a troll and would hardly be the only one curious about how widely applicable the bodyswap thing is...
Hell.
Fuyumi herself tested the matter with her dog. She STRONGLY DISAPPROVES of the idea of Akiho experimenting with other girls though.
They aren't dating, technically, but they kiss because they want to
The two of them do it for pleasure, not pretend usefulness
https://dynasty-scans.com/chapters/swap_swap_ch09#5
https://dynasty-scans.com/chapters/swap_swap_ch19#7
Look, all I'm really saying is that the series itself hasn't said anything definitively either way. And that until it does, Akiho and Fuyumi's relationship is in a bit of a Schrodenger state, and is entirely up to how someone interprets the characters.
Sure, they are effectively already dating, but in many respects so are Natsuko and Haruko. And I can't help but see Fuyumi and Akiho's relationship as being more similar to a more comfortable version of Natsuko and Haruko's than I can to Chinatsu and Miharu's. And I don't think I'm the only one.
So until the series actually touches on it, you're entirely free to disagree with the interpretation, but Chinatsu and Miharu being definitive about it is definitely something novel.
The main difference is Fuyumi and Akiho have already all but confessed to each other
They don't think anything of swapping, or getting naked in each other's bodies, unlike Haruko and Natsuko
https://dynasty-scans.com/chapters/swap_swap_ch19#8
https://dynasty-scans.com/chapters/swap_swap_ch14#10 / https://dynasty-scans.com/chapters/swap_swap_ch24#7
FWIW Akiho actually did get cold feet about following through with her teasing. One also notes the kissing bit was the only thing Fuyumi has ever expressed a smidgen of jealousy over, so it doesn't seem very far-fetched to assume the act holds real emotional gravitas to them both - quite possibly more than they realised before the matter was brought up.
Incidentally, Haruko by comparison... ( . )( . ) ԅ(≖‿≖ԅ)
Exactly, Akiho cares too much about Fuyumi and vise-versa to follow through with the act
That in itself, shows how much they treat their relationship seriously
You could call them girlfriends or lovers, and it would fit better than calling them just friends. They even respect each other's boundaries better than Chinatsu and Miharu