Replace BOTH elements as they BOTH were installed at the same time. NOTHING irks me more than a half assed mechanic going into a home to replace a washer ONLY on one side of the faucet.
Labor is expensive material is cheap. While the Licensed plumber is replacing your elements you will notice some sediment build up on them THIS KIND SIR IS acting like insulation robbing you of money spent for heating. Don’t be foolish have both replaced.
Wouldn’t you feel kind of foolish replacing one now and THEN in 4-6 weeks having to replace the oth