Combos are for an individual to execute and not multiple party members. Meaning that other classes can't add to, or interrupt, a combo you're performing. Also even if a combo's additional effect is resisted, as long as the combo triggering action hits the target, the combo will still be able to be chained together and won't be lost. For example if a combo's special effect is a "Stun", if the stun fails to take effect but your action connects, you can still continue to the next step of your combo.