Can you split a duplex into two seperate units to sell them individually?
Sometimes! In order to pursue this option, you must first inquire with your local registry or board of housing as to whether this conversion will be permitted. You must then alter the LEGAL description of the property (re-deed)- either to 2 condominiums, or to 2 single family attached homes. A consultation with a local real estate attorney will likely give you a sense of what is involved in the process in your are.
If you have a mortgage on the property, you will also need to obtain permission from the lender to alter the legal description of the property (since the mortgage is anchored to the property as it is currently legally described, and will need to be re-written)
Most conventional lending outlets have some additional requirements as to financing recently converted properties for your new buyers- many will want an architect or structural engineer's opinion of the new property and any specialized needs that might have been created by the conversion. Speak with a local lender as to your plans for resale PRIOR to initiating the project so that you can set yourself up for the best possible resale outcome.
This is a case where EXTENSIVE due diligence PRIOR to beginning the process will yield you the best outcome.