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JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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You can manage fields, but I don't think we have an option to retain all fields to both the forms and just create a new form with all the fields combine.
What you can do is to clone both the forms: http://www.jotform.com/help/27-How-to-clone-an-existing-form-from-your-account.
Then use our Field Manager app: http://apps.jotform.com/app/field_manager to combine the two cloned forms.