You will get answers for both, but only one alphabet. Sound good?

Saix:

 A = Aftercare (What they’re like after sex)
He’s not cuddly, per se, but wants to maintain contact with his partner, so he holds them, strokes their hair, talks to them about what just happened. 
B = Body part (Their favourite body part of
theirs and also their partner’s)
On him: His arms
On his partner: their thighs
C = Cum (Anything to do with cum basically… I’m
a disgusting person)
He once came so hard that his cum came out his partner’s nose. He was deeply apologetic, but it was also… interesting
D = Dirty Secret (Pretty self explanatory, a
dirty secret of theirs)
He’s occasionally heard things in the night, the sounds the others make when it’s just them and their subconscious dreams. He liked it. 
E = Experience (How experienced are they? Do
they know what they’re doing?)
Well, he hasn’t done it a lot, but he always does it well. 
F = Favourite Position (This goes without
saying. Will probably include a visual)
He’s a big fan of missionary. He likes eye contact during sex, and that facilitates it. 
G = Goofy (Are they more serious in the moment,
or are they humorous, etc)
Almost frustratingly serious. 
H = Hair (How well groomed are they, does the
carpet match the drapes, etc.)
Very well-groomed. There is hair present, but nothing wild. And yes, it matches the rest of his hair.
I = Intimacy (How are they during the moment,
romantic aspect…)
He’s aware that this is the peak of intimacy, at least physically, and he comports himself as such. It’s super romantic as a result, too (he likes comparing his partner to the moon, it’s kinda sweet).
J = Jack Off (Masturbation headcanon)
He doesn’t let himself… often. 
K = Kink (One or more of their kinks)
He likes shibari.
L = Location (Favourite places to do the do)
Under a window, in the moonlight. 
M = Motivation (What turns them on, gets them
going)
He honestly isn’t much into it until his partner is. He waits for their cue, gauges their reactions, their needs. 

Demyx:

N = NO (Something they wouldn’t do, turn offs)
He doesn’t want to do breath play, ever. 

O= Oral (Preference in giving or receiving, skill, etc)
Very slightly prefers to give, and why wouldn’t he? He’s very good at it. (ADJOINING HEADCANON COURTESY OF FLURRY: he got good by practicing on fruit, especially pomegranates)

P = Pace (Are they fast and rough? Slow and sensual? etc.)
Relatively slow, normally. “His mood affects his pace and/or spot on the gentle to rough scale” is probably the best explanation.

Q = Quickie (Their opinions on quickies rather than proper
sex, how often, etc.)
They’re good when he’s desperate, which is rare. He’s generally pretty okay with waiting a little while until there’s enough time for a solid session. 

R = Risk (Are they game to experiment, do they take risks,
etc.)
He likes to experiment within the bounds of safety (kinks are fine, but nothing too dangerous)

S = Stamina (How many rounds can they go for, how long do
they last…)
(already answered, see bottom of post for extra answers)

T = Toy (Do they own toys? Do they use them? On a partner or
themselves?)
No, but he wants some, for use on both himself and his partner

U = Unfair (how much they like to tease)
Only a little. 

V = Volume (How loud they are, what sounds they make)
(already answered, see bottom of post for extra answers)

W = Wild Card (A random headcanon)
Not solely sexual, but he likes coconut-scented (and flavored!) things.

X = X-Ray (Let’s see what’s going on in those pants, picture
or words)
Also from Flurry, and all I’m gonna say: 8.5 inches

Y = Yearning (How high is their sex drive?)
It’s less about general sex drive and more about desire for his partner. That said, he’s got what’s colloquially called a healthy sexual appetite. 

Z = ZZZ (… how quickly they fall asleep afterwards)
He needs to fully calm down from all the excitement before he’d even think of falling asleep, and he likes looking at his partner while they’re all lit up with afterglow, anyway, so it takes him a while to fall asleep after (which is great if his partner likes to talk after) 

Extra answers: 
C = Cum (Anything to do with cum basically… I’m a disgusting
person)
He kissed his partner once after they’d gone down on him and was kind of pleasantly surprised by the taste of himself on their lips. (Note: he kisses them after oral– regardless of who’s giving and receiving– all the time anyway; this wasn’t new, this is just the first time he noticed)
H = Hair (How well groomed are they, does the carpet match
the drapes, etc.)
You’d think his fastidiousness about his hair would apply everywhere on his body, and you’d be wrong XD. Also the carpet is the same color as his roots, not the tips of his hair (#blondproblems)