Both actors will appear in the episode side-by-side. According to the story, Takei is to appear as the father of Simon Templeman's Larry Bird, who has come to Earth with an important announcement. Hamill will portray a co-worker. As can be seen in the image below, they're obviously in the same scene together as opposed to their working apart in the same episode.
And for those who have NOT seen The Neighbors - about a neighborhood of aliens who look like humans, and have one human family in their community - it's highly recommended that you check it out. It is one of the most bizarrely enjoyable series on the air.