It's not a sequel at all, and in no way do I imply such in the letter. It's a mini-bio, in which I state that I've written a number of short stories, this novel, and am a third of the way through my second novel, BLAH. No? Not a good approach?
ETA: Now I'm worried. How would saying that be an invitation to reject me?
I guess I'm still a little confuzzled. Your OP said, in part,
CaroGirl said:
I'm sending a cover letter, with my request for a partial, which includes a short bio.
To me, 'cover letter' and 'request for a partial' indicate that you are sending requested material as a follow up. At that point a cover letter says, essentially, "Here's the stuff you asked for. See attachment at Tab A."
From your later post, it's looking more like a query letter, with a bio (or cirriculum vitae), a short synopsis (who, what, where, when, why, and how), and an attachment of three to five pages as a writing sample. In that case as well, I'd only mention a second work in progress as simply that, a work in progress. "I'm shopping around my recently completed 500,000 word chick lit novel
And How Do We Feel Today. I love puppies, I've been published before (cite1, cite2, cite3), and I'm working on other pieces even as you read."
Please note that I am neither an agent, a publisher, or anyone who has ever submitted a written work to either of those beings.