Im loving this idea:
A generation ship is sent from earth with 1,000,000 passengers and crew towards a planet 500 years away.
http://en.wikipedia.org/wiki/Generation_ship
And then another one is sent after them 300 years later, with another 1,000,000 people onboard
http://en.wikipedia.org/wiki/Cross_generation_ship
The second ship meets the first ship 50 years after it launches, 350 years after the first ship launched. They link up to each other with tethers, and then meet up by using short range shuttles and then setting up tunnel tubes between the ships.
They have an uncomfortable reception, 6-7 generations have passed on the first ship, and none of its inhabitants have any memory, or any fondness for earth, the world that exiled them to a life of loneliness.
The second ships advanced crew and passengers are seen with envy and distrust by the first ship, and to that crew, the first ships inhabitants are uncultured idiots, unfamiliar with modern technology and unwilling to learn it.
Hostility and fighting soon breaks out as the two communities mingle, around 200,000 personel from each ship are killed when the fighting escalates, biodomes cracking and freezing everyone inside them, oxygen and fuel tanks rupture, engines crach, and the ships are nearly crippled.
The two communities then fall back to their own ships, anyone left from the other is excecuted on the spot.
The ships have no choice but to stay on the same course, the more advanced ships engines were destroyed in the fighting, and cant jump ahead of the other ship now that they have matched speeds for the link up. Its impossible to change direction, as each ship was designed to get to one target and one target only.
For 150 years the two ships sail side by side through the stars, for 150 years the coldest of cold wars endures, each side training up its people for warfare. Only occasional incidents take place, often only in retaliation against imagined spies or equipment failures blamed on sabotage.
When the 500 years are up, the target planet is in sight and the ships prepare for the colinization of the planet.
However, things do not go smoothly.
The first ship was designed to orbit the planet and drop biospheres down onto the planets surface, while the second ship was created with the ability to land, intact onto the planets surface, from which it could act as a centre of living and expansion.
The ships are still tethered together, and the tethers have been fused to each ship, as 100 years ago the two ships tried to detach from each other, but only succeeded in melting the tether points and welding them together stronger.
The ships enter the planets atmosphere, the second ship dragging the first behind it. The landing is catastrophic, the second ship acts like an anchor, touching down first and being dragged around an 8th of the planets diameter, until the tethers pull out huge sections of either ship, the first ship then catapults off, landing 25,000 miles away.
The survivors of the wrecks then step out onto the planets surface, and see their first sky and horizon, blackened by the fires from the crashes and scarred from debris from the planetfall.
500,000 from each ship lie dead in the wrecks, the survivers are now out for vengeance, blaming the other ship for detroying their families, homes and lives.
Each ship had stores of weapons and vehicles designed for war (incase of native resistance), and the survivors break these out and start to advance over the surface of the alien world, trying to claim as much territory as possible before the other ships people do.
