Common sense dictates that, if they are getting two houses for the price of one, then you'd have to give two appraisals for the price of three, because two plus one equals three. That's true even in Las Vegas, the fertile valley.
If the first house is full, market value price and the second house on a separate lot is thrown in as a sellers concession, one 1004 (assuming it's a financed deal) would suffice. Just don't forget to comment on the additional house as part of the PA. Based on these terms and conditions of the PA (2 for 1), these sale would never qualify as a comp. Imagine taking a -$150,000 adjustment on a $150,000 sale. I suspect sellers motivation disqualifies this as a comp.