… interesting

Uh-oh

Incest? Hell Yeah!

Incest is wincest

Aren't the eyes a bit of a giveaway though? Since Spike still has green eyes despite the swap?

Spike really is going for broke here, ain't him? Go get 'em! Although I do believe he should practice with Rainbow first. If only to let her get a taste of his grapes

The fact that the first thing Twilight did with her double cannons was not giving Spike the double penetration of his life is rather disappointing. For shame Twily, for shame!

Well, here's hoping there are no negatives feels about this. Get the feeling, Twilight, is about to be a HUGE prev here, and Spike's counters are about to be a little pre-mature.

Good writing and good luck!

"...fine, you've got me there. You're just so damn fluffy, it's awesome." Spike smushed his cheek against his foreleg as he said this, a small buzz in his body as he hummed in appreciation. "How do you not just rub your fur all day?"

Now I want a pone to snuggle for myself!

Twilight shook her head. "The power should still be there, you just need to be attuned enough with me in order to draw it out. Its like how you had to grow accustomed to standing and walking as me at first, it should be easy once you get a hold of it."

Wouldn't it rather be that he has the magical power of an alicorn combined with the magical control of a foal?
That would be a very dangerous combination...

They were patters of affection that landed against the most pleasurable spots of his body, pressure points he never knew he had, for he had never had them.

Almost as if the knew his body like her own?
If you think about it that actually makes lot of sense.

"Are you even sure that the duplicate is completely capable?"

: "Only one way to find out."

"Sweetie, Scootaloo, and Apple Bloom?" Twilight asked, a curious glint in her eye. "How old are they these days, again?"
"Sweetie and Apple Bloom are both nineteen, Scoots recently turned eighteen. Why do you ask?"
"No reason! Anyways..."

I hope she doesn't think what I think she is thinking.
(And now I feel bad for thinking about it in the first place.)
Edit: It looks like both are thinking about it...

Woah, that took a huge fucking left turn there at the end