Something like this could work:
Begin Date Compare : [>{CMDWHENISAY}pressTimeout] Is Greater Than Current Date/Time
Set date [~holdTimeout] to [~holdTimeout] plus [150] milliseconds
Start Loop While : Keyboard Key 'Ctrl' Is Pressed
Begin Date Compare : [~holdTimeout] Is Less Than Current Date/Time
Write [Blue] 'Tap and hold registered' to log
End Condition - Exit when condition met
End Loop
Else
Set date [>{CMDWHENISAY}pressTimeout] value to the current date/time
Set date [>{CMDWHENISAY}pressTimeout] to [>{CMDWHENISAY}pressTimeout] plus [500] milliseconds
End Condition
The first compare checks whether the Date value stored in ">{CMDWHENISAY}pressTimeout" (the "{CMDWHENISAY}" token is used to make the name unique, so that multiple commands won't interfere with eachother) is within the time limit for registering a double tap (by checking whether the point set in the future previously is still in the future).
If so, a second value is set 150ms into the future, after which a loop checks whether the Ctrl key is held down. Inside the loop, a condition continually checks whether the 150ms have elapsed, and if so writes "Tap and hold registered" to the log (replace this with the actions you want to use).
Once that is done, the command will exit (to stop the loop; Otherwise chosen actions will continually run while you keep Ctrl held down).
If the Ctrl key were to be released before the 150ms have elapsed, the loop ends without the condition inside it executing its actions, and the command comes to a natural stop (by reaching the end of the action list).
If the first compare returns false, meaning either the key has not been pressed yet, or it hasn't been pressed within the last 500ms, ">{CMDWHENISAY}pressTimeout" is first set to the current time, then 500ms is added to it to define the point in the future beyond which a double tap will no longer be registered.
For a command like this, you could add the "Ignore an Unrecognized Word or Phrase" action at the top of the action list, so it doesn't show up in the log when you press the key (no log "spam" that way).