Two ‘directions’ in form

I'm creating a form that has a 'split' section, where the user either has to fill in 'Section A' OR 'Section B'.

These sections are quite large, with around 6-7 fields in both.

Another requirement I have for this form is that it needs to be on one 'page' (one webpage, not fit on one screen height).

I have looked for duplicates of this question, but can only find questions in relation to one input, not entire sections being 'split'.

I've created a few mockups of ideas I've had:

Mockup 1

This has the advantage that the user will be able to see every field before they decide which one to enter. Although it may be confusing; for example, is the generic field at the top part of it? Would users know to only fill in one section? If I disable the 'other' section, then what if they change their mind?

mockup 1: 'OR'

Mockup 2

Firstly, apologies for the bad mockup. This looks much tidier, but hides the second option. This may be a problem for a few reasons:

  1. The user has to click the second tab to see the fields.
  2. They may not know to only fill one in, though this could be solved with some text saying 'only fill in option 1 OR option 2'.
  3. The user may attempt to fill in both, that would raise the question of which do I accept?

mockup 2: 'tabs'

So my question is:

Is there a general rule/protocol for when this problem occurs? Any advice?

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