By current design :
1) Onsubmit expects a single function and it should be one for form. However, while submitting the request, it will aggregate Onsubmit values of all forms inside that request. In this process, it will not allow to submit, even if there is failure of Onsubmit in one form.
2) you can have multiple onLoad functions separated by ';'. But this is not the case with 'onSubmit'.
So onSubmit attribute of a form , you can ONLY place one js function there .