How do we determine the basis of individual lots purchased together for a single price that need to be treated individually when sold?

Just over a year ago we bought two lots for a combined total of $40,000. The verbal agreement with the seller was $19,000 for one and $21,000 for the other one that we are now selling. Our Realtor wrote it up for the total $40K price. All of the closing document list both lots and the $40K price with no price breakdown. We have a buyer for the 19K lot and we need to know how this will be treated for next years capital gains taxes.  When we tried to do a square foot calculation they came out at $28K and $12K. How do we determine the basis of the lots that need to be treated individually when being sold?