Only way to legally split a property is to have it surveyed and “parceled” off into two separate tax parcels with the county recorder’s office. If you have a mortgage against the property now, you won’t be able to divide it and sell half, unless in doing so you payoff the mortgage in full. The proximity of the houses might or might not be a problem, depending on local zoning ordinances, so check with the county zoning commission first!
I would first talk to the zoning administrator and have him or her take a look at the property to see if it is feasible and meets zoning requirements for site area and set backs.
From there you will need to get a survey and two legal descriptions.
Since your legal description has changed (and maybe your property address), you will need to refinance. This should not be a problem if you use the sales proceeds to pay down your first mortgage. The low interest rates makes this a good option as well.