This is just me bein a derp but i like thinking about why this would even happen. I do so with no form of context of anything beforehand at all so there you have it.

I've always liked Super RD because it brings around the idea that she and the others have a potential second form based on their elements. Twilight is the exception here because she had to realize her element to partially activate and assign the others in the first place. (that and she's pretty op with her alicorn form now but this was of course created before that)

Rainbow Dash would most likely discover her's first due to her being a shining example of pure loyalty when her friends are in serious trouble. For the sake of making this conflict sting more, let's say it would be when Twilight is against the ropes fighting a really powerful enemy. RD's strong desire to help Twilight mixed with her brash impulses would trigger her element, forcing it to go into overdrive to support and protect her during her defense of Twilight thus bringing forth Super RD.

After everything is said and done, Twilight goes through great lengths to research this phenomenon and over time RD learns to control the transformation. However, it still takes a serious toll on her requiring Rainbow to rest (more than usual) afterwards. Of course with all the research, this means that Twilight and RD have been spending an unusual amount of time together which wears on both of them. Rainbow Dash tends to break anything and everything during testing much to Twilight's annoyance and RD would much rather be off on her own relaxing without Twilight constantly being in her face about this or that.

These tensions gradually grow until eventually Twilight moves on to trying to induce another one of the elements' super form, namely Fluttershy. At first it seems as though this should be easy as Fluttershy seems to never be at a loss for some form of kindness. As the days roll by with no progress in sight, Twilight starts to lose her patience while becoming more and more abrasive to Fluttershy.

Alot of this behavior is due to Twilight being frustrated that she hasn't been able to induce a form of her own, not yet understanding why, and hating that despite all of her progress power wise, she still needed help from RD when it counted. Twilight has always had a pretty short temper to begin with, and with all of these factors piling up she finally snaps. At first she seems to just be a bit batty ala Lesson Zero. She tries really bizarre and almost terrifying methods in order to try and cause Fluttershy's transformation all the while causing considerable damage to the surrounding forested area. Then, when it seemed like Twilight was about to turn her attention to Flutters herself, Rainbow Dash who had been investigating the sudden commotion from above swept in to Fluttershy's defense.

This doesn't do wonders for Twilight's composure at all as her crazy turns into an intense rage that literally has her seeing red.....red eyes....a fiery form...she could do that too y'know. And with that, Twilight channeled as much magic as she could bringing forth a familiar form that she had only done briefly once before. Despite, or maybe because of her currently intense but unstable state, she was able to maintain it. Fire Twi finally vents all of her frustration at Rainbow telling her everything and throwing in a heavy amount of insults her way. Most of the time, Rainbow had a hard time taking Twilight seriously which would result in her ignoring Twilight's smaller plights and tirades. This time around however it was as if Twi just knew every button to push to anger her. Eventually, Twilight demanded that she moved so that she could continue her "work" with Fluttershy. Hearing her childhood friend's whimpers behind her, and feeling the literal intense heat from Twilight's rage in front of her, RD had no other choice. She transformed once again into Super RD and prepared for the fight of her life.

P.S. When it looks like both of them are on the edge of falling, Fluttershy's super form finally activates in order to amplify her stare ability to keep RD and Twi from continuing their fight for fear that one or both of them may die. Once they break out of their super forms, she finds a way to use the magic of her element to tend to their wounds until they properly recover. Although they all eventually reconcile with each other, Celestia was surprisingly caught off guard by these events and in turn has all three of them put under closer surveillance.

^it's dumb, it's a mess, and it's passed 4am so imma go sleep in the corner now.

awesome

Rainbow Dash:
"Read dat, ya frickin' coward!"
"That fancy scope of yours, bet ya got a REAL good view of me readin' ya!"
"You looked da whole time for dis?!"
"It was a mercy killin', ya live in an... air balloon!"
"You'll never hit me! You'll never hit my tiny head! It's so tiny! I got a frickin'... such a tiny li'l head!"

Twilight Sparkle:
"Ya rainbow ankle-biter!"
"That'll slow yer down, ya twitchy hooligan!"
"Ya miniature delinquent!
"Couldn't skip around that could ya, ya precious little pony!"
"Ya spastic little gremlin!"

everypony would die

Twilight's getting a big advantage soon.

How is this possible to decide when both are awesome.

(COmment below) Collision of these forces would cause more than equerstia to be wiped out.

wow... if those 2 forces would collide... equestria would be in pieces

Who will be...

THE DEADLIEST WARRIOR!

Super Rainbow Dash vs Rapidash Sparkle!!

awesome i always wanted to see something like that