You apprasing the real estate or the business?
If you are appraising real estate, you'd compare with other retail buildings similar in size, location, etc. for the market and income approach. The monthly rent $$$$ is what counts more, not the tenant's retail use.
Assuming that you are appraising the real estate only the first question is are both stores on one parcel of land? If they are then you might want to look for multi tenant commercial buildings. If they aren't then you have sales of two properties in one transaction. Assuming that they can be sold seperately I would appraise each separately.
Look real carefully at the allowable use and zoning. I have seen areas where only a small area is zoned for liquor stores and as such command a much higher rent than the typical retail space. The GI to the business for a liquor store is significantly higher per SF than the typical small retail outlet due to the product, price per unit, etc. so a rental that includes a base and percentage based on profit can generate a much higher GI/NOI to the property owner than the auto parts store.
I believe I would try to get sufficient income rental info to make an income adjustment. A NAPA store could be an owner-operator, but often Auto Zone, etc. are leased from absentee owners who were contracted to build the building and have favorable 5 year or better leases. I might try to find some auto parts stores like it and then add a contribution via income adjustment for the Liquor store. Most sales of Liquor stores, in my opinion, are affected by the gross sales...ie.-sell as a business that is difficult to seperate from the real estate.
Fix yoru car in the parking lot of the Napa and down a few cold ones while doing so.
I should have mentioned this in my first answer. If you are located in a state with complicated liquor laws, like New Jersey, the fact that the property has an active liquor license may add considerable value to it.
In this area liquor licenses are limited by population as well as distances from schools and local regulations. If the subject property is the only legal location for a liquor store or a grandfathered use it is worth more money.