Try to grow a second tail

Sophie: "Let's say I want to try a Tails impression. If I can turn this fat slab of meat into a fly swatter, surely I can branch it in two."

And so you do; easily, in fact. Your tail is now fork-shaped.

Sophie: "Okay now if I just..."

You grab either end of the forked branches and pull them apart. Your tail doesn't seem bothered by this forceful transformation; It's like putty in your hands. The split elongates, until it hits your pelvis. Ta-da, now you have two tails! They are slightly more unwieldy to control, but not impossibly so. Most importantly: both of your tails have the same amount of control and malleability as your original.

Sophie: "Okay, let's try an upgrade to a kitsune... Nah wait I'm a cat not a vixen. A catsune then."

You straighten your back and focus intently on your spine. It tingles, sparkles with magic, as you will one of your tails to split twice, then those two twice, and those four once more, giving you a grand total of nine.

These brand-spanking-new protrusions that decorate your backside also contain nearly half of your body weight. This makes even walking arduous. Thankfully, with a bit of further concentration you're able to change the tails' density to nearly non-existant. While still packing considerable prehensile strength, your tails are now literally featherweighted. Your flowing, firm furpoles dance underneath your touch, reactive like snakes.

Sophie: "The things I could do with these... I'm kinda like an octopus, but even stretchier and more versatile! I could do all of my housework at once. I could turn them into enormous pythons for that added intimidation factor. I could even do some super lewd stuff with these... imagine being the first person in the world to try nonuple penetration!"

Greed begins to overtake you. You try splitting your tails even further. While highly successful, after a twelve or so tails all a wrist's thickness, your spine is starting to feel crowded. When, in spite of your best judgement, you decide to pump out one more appendage, you hear a nasty CRACK.

Sophie: "Eep!"

Unlike what you'd expect, there's no pain whatsoever. No blood either. Maybe you destroyed your spine and are paralyzed? Surely not, you'd have keeled over by now.

Sophie: "Ummm."

Did you spine just... split along your lower back? As if that's a thing that it can do? You need to test something.

Grabbing a butt cheek in each hand, you tear your entire body in half, all the way up to your skull. Still, no pain, no blood - your insides appear featureless and uniform. Your head splits in two like a ball of marmelade, leaving both halves of your brain still fully cognizant, and in complete control of their respective body halves.

You - both halves of you, re-form their missing body hemispheres from nothing. Now you have two bodies, with seven tails each. You get up and feel up each other's bodies.

Left Sophie: "Holy balls! I didn't even realize I was this hot."

Right Sophie: "Better believe it, sis. Look, I wanna fuck my own brains out too, but we gotta experiment a little more first, okay? Mmmaybe we'll find ways to make our lovemaking more interesting?"

Left: "You read my thoughts."

Right Sophie: ;)

What you discover is exactly what you expected: Your entire body is capable of the same sort of transformations as your tail. You can shapeshift into any form, creating new limbs and in fact any kind of proptrusions at will. More complex ones require more concentration, but you're able to, for example, grow five extra legs, give yourself literal octopus tentacles, cut off large portions of your body and re-introduce them back harmlessly, and, most importantly, create seemingly infinite amounts of matter in the form of your own body parts.