Very simply, in Valmiki, nothing is written about Kush or Luv after Sita's bhumiprasthan, aside from their coronations. Rama took them and continued the yagna, and for 1000 years after that, he did an annual yagna, presumably to atone for the treatment of Sita.
After several years, the 3 queen mothers KSK died in succession, and Bharat's uncle Yudhajit invited Rama to conquer Gandhara on his behalf. Rama sent Bharat & his 2 sons Taksha & Pushkal to conquer the territory, while after that, he, Lakshman & Lakshman's 2 sons Angad & Chandraketu invaded Karupadhadesh (on the foothills of the Himalayas) and conquered that. Bharat's & Lakshman's sons were given their own city kingdoms.
After that was the Durvasa incident which resulted in Lakshman's abandoning the world, and following that, Rama decided to abdicate the throne & follow him. He offered the throne to Bharat again, but this time, Bharat would have none of it, and instead suggested that Kush & Luv be given North & South Kosala. The two of them were crowned in Ayodhya, while in Mathura, Shatrughan too passed his kingdoms of Mathura & Vidisha to his 2 sons Subahu and Shatrughati.
That was the last mention of Kush & Luv in Valmiki's Ramayan.
The accounts of Kush - his marriage to Kumudhati, his battle w/ a rakshasha that ended in both their deaths - is all a part of Kalidasa's Raghuvamsa. In other words, it's very possibly poetic imagination, or in other words fiction. None of them were married while Rama was still alive.
Bharat's sons ruled Gandhara - their capitals were Takshashila (Taxila) and Pushkaravati (near Jalalabad)
Lakshma's sons ruled Karupadhadesha - their capitals were Andavad & Chandrakant - likely in Nepal.
Shatughan's sons had capitals in Mathura & Vidisha.
Kush got Ayodhya. Some accounts have Kush's capital as Kushavati & Luv's as Shravasti. But given that no account is actually given of Luv himself, it's possible that Kush got Rama's kingdom, while Luv got Dakshin Kosala, which is today's Chattisgarh.
Edited by .Vrish. - 06 December 2011 at 9:51am