That shouldn't be a problem at all. You can definitely run many concurrent tests on a site. In this case you're running Test A (current test) and Test B (form optimization test). You'll have:
Landing Page
- Test A Control code
- Test A Sections
Signup Page
- Test B Control code
- Test A Goal code
- Test B Sections
Confirmation Page or Submit button click event
The existence of Test A's goal code on the signup page won't make any difference. GWO uses a modified version of Google Analytics, and basically every test is a separate account. They shouldn't create any conflict with each other.
One note: I've not run separate instances of the control code on the same page, but I think that's safe, too.