Thanks for your question. Yes, per the rules you may enter multiple submissions into the Sparkathon.
"(vii) Multiple Submissions: A Maker may submit more than one Submission, however, each Submission must be unique and substantially different from the others, as determined by the Poster and/or the Administrator."
Please note, however, that existing applications are not eligible for prizes in the Sparkathon. Per the official rules: "(iv) New Applications: Applications must be newly created by the Maker after the start of the Hackathon Registration Period (October 8th, 2015) and may not have been entered into a previous hackathon or competition."
There are two ways to enter additional submissions to a competition.
The first way is to add a second project to your portfolio. When you begin the project you'll be asked if it is part of a hackathon. Select the hackathon you'd like to enter your project into from the list and it will add it as a submission.
The second way to add a second project is via the competition homepage. After you've entered your first submission (meaning that you've clicked "Submit to the Sparkathon) then you can start another from the competition homepage. To do this, submit your first entry, then navigate back to the hackathon homepage. Under the "Edit your submission" button you'll see a new link "Or start a new submission."