Can I merge two user records/accounts?

Short answer: No.

Longer answer: 

Pick one of the user records and set Login Active, Current, and Contact Active all to No. You typically pick the record that has been least used (in terms of assignments, timeslips, etc.), or has not been used recently.

Each user has a contact record, and if desired you can merge those contact records.

If desired, you can transfer some information from one user record to another. See Mass Assignment Updates and Mass Calendar Updates. If you do this, it is helpful to at least temporarily change the name of the user you are inactivating. Changing Jane Smith to Jane Smith(inactive) will help when selecting the users during the update process.