Merging the accounts directly is not really possible due to how the encryption works. Probably the most feasible thing to do would be to export the data from the account you want to get rid of, and import it into the other account.
Export instructions: https://support.lastpass.com/s/document-item?bundleId=lastpass&topicId=LastPass/Export_Encrypted_XML.html
Import: https://support.lastpass.com/s/document-item?bundleId=lastpass&topicId=LastPass/Import_LastPass_Data.html