I believe you should be able to do this with conditional rules on the form processing steps. Are the two forms on completely separate pages are is it the same page?
If it is two pages (i.e. www.example.com/register and www.example.com/profile) than just create a hidden field that is set to "Create" for register and "Modify" for the profile page and use that in your conditional steps.
If it is the same page it would need to be a bit more intricate but really you should be able to do it with some simple JS and web data lookups.