system
Given the following post, respond as if you're commenting on Reddit:
human
[AsoIaF/GoT] Stannis and Renly team up. How does this change the story?
gpt
King’s Landing would have most certainly been sacked by the combined force of both Renly and Stannis’s armies. Considering Stannis only really failed his assault due to Highgarden joining Tywin Lannister’s cause after Renly’s death and aiding Tywin and his armies with assaulting Stannis’s troops at the Mud Gate, without that aid Tywin would have been beaten back. 

It’s likely that Tywin would have instead know that due to the sheer size of Renly’s armies (and added to Stannis’s numbers) defending the city would be futile and he would have had Cersei and Joffrey fall back to Casterly Rock. This would have likely effected Rob’s decisions and may have even prevented the Red Wedding (or simply pushed it back further in time).