Probably not quite the dumbest idea I've ever had... but I'm sure it ranks pretty high up there...
Teppelin City, a metropolis amongst the desert.
Four years ago, a brave band of rebels defeated the tyrannical Spiral King and liberated the human race that had been forced underground.
Human-Beastmen racial relations are still tense, but the remaining leaders on each side are making strides toward a peaceful coexistence.
But recently, a string of mysterious attacks have been taking place, targeting civilians of each group. Surviving victims claim to have been assaulted by monsters that were neither human nor beastmen...
In a deserted parking garage late at night, a young man is fiddling with a device in his hand. It appears to be some type of a USB stick.
Standing across from him is a blue-haired individual wearing a longcoat. "It's about time we managed to track you down, Dopant asshole."
The first man grins, and shoves the devices into a port in his hand. "It's only been two days. I'm impressed that you already found me... Simon the Digger."
Energy from the stick flows into his body, transforming him into a humanoid monster made of stone.
"You can credit Rossiu for tracking you down. Me? I just do the leg work." Simon retrieves a device, and attaches it to his waist, forming a belt. Then, he retrieves a USB stick of his own, and inserts into a slot in the device, causing a voice to ring out.
LAGANN!
Then, another USB materializes into the second slot.
GURREN!
"Henshin!"
As the device then splits into a W-like configuration, Simon is covered in a swirl of flame.
GURREN LAGAAN!
In Simon's place, is now a costumed figure with a biker helmet. His entire right side is colored a blazing, heroic red. His left, midnight black. Two gigantic eyes in the helmet start glowing green.
Dramatically pointing at the sky above with his finger, the the masked figure declares, "Two detectives in one, Kamen Rider Gurren Lagann! Just who the hell... DO YOU THINK WE ARE?!"
So. TTGL meets Kamen Rider Double. Garbage? Epic? Epic garbage? You decide.