De man 5 crontab
:
Step values can be used in conjunction with ranges. Following a range with
''/<number>'' specifies skips of the number's value through the range. For
example, ''0-23/2'' can be used in the hours field to specify command
execution every other hour (the alternative in the V7 standard is
''0,2,4,6,8,10,12,14,16,18,20,22''). Steps are also permitted after an
asterisk, so if you want to say ''every two hours'', just use ''*/2''.
Portanto, /2
deve seguir um intervalo de valores. No seu caso, você quer executar o comando a cada duas horas no intervalo 1-23, então 1-23/2
funcionaria.