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.
You shouldn't be placing double-quotes around your statement like that. It's likely to be treated as a literal. 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.
EDIT:
Also just noticed, you shouldn't be placing double-quotes around your statement like that. It's likely to be treated as a literal.