I can think of two approaches:
(Option 1) Schedule the automation to run every Monday. Have a scripting action that calculates if it is the 2nd or 4th Monday and outputs the result. Have rest of the automation in a condition based on the script output.
(Option 2). Have a control table with two records. One record for the 2nd Monday of the Month, and another record for the 4th Monday of the month. Use a formula field to calculate the actual date. Use a “when record meets conditions” trigger.