Why would a starship need that?
The engineering chief looked up at the Captain from where he was half-kneeling, uniform covered in lubricants, glues, solvents, paint, and several stray bits of silvery, adhesive tape. His hands were calloused and filthy, and a few grease smudges discolored his face and mussed hair.
"Sir?" he asked looking perplexed.
"I said, Cheif, this is a warship - so would you kindly tell me why we now have a second warp drive installed, and plugged into the first one no less? For that matter, how the hell did you even get it in here? And when, from whom!? I never authorized this!"
"Well, cap'n, I had it brought in during our layover at Vulcan and we've been assembling it while the crew rotated out on shore leave." The engineer had replied in a deadpan tone, his face totally unemotional.
"Why!?" The captain demanded, practically shouting.
"Well ya'see sir, I have this theory, and to test it I needed a second warp drive. The Vulcans probably just assumed it was for spares - what with the war with the Klingons and all - and you were so busy catching up on paperwork, I didn't want to bother you with my hobby."
"Hobby. You call a massive engineering overhaul to test a theory a hobby."
"Aye sir." The Captain sighed, placing both hands over his face.
"And what, pray-tell, is this theory of yours?"
"What do you mean it worked?" asked Professor Telvar as he walked through the vast halls of the Vulcan Science Academy. At his side walked Starfleet admiral Bredell, an Andorian with the telltale blue skin, white hair, and short antennae that identified her species.
"The humans attached two warp drives together to see if the ship would go twice as fast!" She laughed with outraged incredulity. Telvar's slanted eyebrows rose higher on his forehead than the tips of his pointed Vulcan ears.
"Your amusement at such a reckless experiment upon a Federation starship during a time of conflict is highly illogical, admiral. Would I be correct to assume that fitting discipline had been applied to prevent such dangerous activities in the future?" the Vulcan asked in his steady, mellow voice, untouched by any emotion whatsoever.
"Well, yes and no. The pink skins, you know, they can be strange, insane even, but more often than not it works out well for them."
"Indeed. The human propensity for foolish endeavors and their irrational belief in luck is well known. As, I must add, their reckless disregard for their own personal safety and survival all too often. However, you haven't answered my question."
"Right, so Starfleet command couldn't let such recklessness go without a response, but since it worked so well we couldn't just drum him out if Starfleet or Court Marshall him. So a compromise was reached - the Cheif Engineer was promoted and he, and all his accomplices, were transferred to Starfleet Engineering back on Earth."
"Curious. I don't understand the logic in such reactions. But what if the experiment? You said it worked? What was it?"
"Oh, right! Well, about six months ago the USS Zephyr stopped here at Vulcan for short leave, minor maintenance, resupply, that kind of thing. Well the Chief has a warp drive - an entire damn warp drive - created on board in pieces, then he and his engineering crew installed the damn thing without bothering to tell they're Captain! And the best part, the funniest, craziest part? He connected it to the already existing warp drive... to see if two warp drives would make the ship go twice as fast!"
"That... sounds... highly implausible..." The Vulcan stammered.
"No, the implausible part was when they hit warp nine point six without tearing the ship in half."
"I think... Yes, I think I should tell Ambassador Sarek this. It may be if some value to him."
"The Ambassador will be part of the delegation to the Klingons."