I agree with others above and try to get a fresh story out, and not a sequel. Why? Because if and when you get an agent, they will make editorial changes to your work. Then, if and when you get a editor at a publishing house, THEY will also suggest changes to your work. It might be minor, or the knock on consequences could mean the plot/tone/characters whatever in your sequel will need to be completely re written or trashed. Plot by all means, but most agree starting a sequel before the first book is placed is usually counterproductive.
Also, again just my experience, but I find concentrating on getting an agent MUCH easier if my heart isn't wrapped into the manuscript. The only way to do that, to have a professional head and not go insane, is fall in love with a new story
