You can't sequence spells onto different targets without modifiers. The misdirection part is fine but the Hunter's Mark part should either be a separate keybind or have a modifier button.
Or if you're more of a clicker and want to have Hunter's Mark on rightclick.#showtooltip
/cast [mod:alt]Hunter's Mark;[@focus,help,nodead][@pet,nodead]Misdirection
#showtooltip
/cast [btn:2]Hunter's Mark;[@focus,help,nodead][@pet,nodead]Misdirection