Ok, can someone buy two lots or two houses and build a house on top of them or do they have to buy one lot or house?
Of course you can buy two lots and combine them. There would be two tax bills until you asked the county to combine the tax numbers. You couldn't build between the properties if there is a utility easement. I think a lot of legal fees would be involved.
Yes you can ,
however realize that if you are buying 2 lots and then combining the lots for 1 house as an example the entire package is worth less in terms of resale for the simple reason that you now have 1 property as apposed too 2.

A good idea here if possible is too build the new house on one property only & use the second property as well.This way if the need ever arises for $ you are free to sell the second lot at a profit.
Hope this helps !

Realtor