What is the optimal layout for a form with two respondents?

Question

^{download bmml source – Wireframes created with Balsamiq Mockups}

I'm redesigning a form that is used by office workers who interview respondents in person. In some cases, there will be two respondents.

The current design is (2). My suggested approach is (1).

Notes:
a. The interviewer usually asks each question to each respondent in sequence
b. This is (unavoidably) a long form, with many questions
c. Important: each question may lead to a subsequent question -- depending on the respondent's answer. Follow-on questions are hidden by default, but displayed progressively. Thus, respondent 1's first answer may lead two "new" follow-on questions, while no follow-on questions may be displayed to respondent 2.
d. I will conduct usability testing but for now I'm looking for guidance

Answer 1

One problem with option 1 is that with a long list of similar looking response types, it might be possible to confuse the respondent's answers between questions, and it won't be so easy to compare if you have onward answers from one respondent only. The upside of option 1 is that for long forms there is less side to side movement of the mouse/arm/hand [depending on device] which may be significant for repeated use

Here's another possible approach (number 3) into the mix:

For questions asked of both participants, span across both columns, but above the answers.

Where an answer leads to one participant having further questions then include that interaction in the appropriate column as in the mockup below

This gives a good side by side comparison. If you feel it's useful to be able to scan down a long form and see where responses differed, then perhaps consider some additional automated indicator of whether the answers match each other, for example in the mockup below, using a tick or cross in above the line that divides each respondent's answer.

Although, you might want to use icons that don't make it look like it's a right or wrong answer!!

^{download bmml source – Wireframes created with Balsamiq Mockups}