You should prorate the selling price between the two parcels and mark each as "Sold" in the MLS.
Example: You sold a house for $200,000 plus an adjacent lot for $50,000, so the combined list price for the two adds up to $250,000.
$200,000 / $250,000 = .800, so the house represents 80% and the separate lot represents 20% of the total listed value.
The properties then sold to a single buyer who offered $220,000 for both. For the house: $220,000 x 80% = $176,000 and for the lot: $220,000 x 20% = $44,000. Adding the two (or more) prorated prices will thus add up to the total sale price - in this case $220,000.