The use of && is actually a shell operator meant to be interpreted by bash. This will not work in cron. Generally speaking, if you want to run multiple commands in a single cron task, you really should put them in a script.
If you are truly intent on running them inline, you can do something like the following: bash -c 'whoami && which whoami'
But again, at that point you should really be placing your commands in a shell script.