Yes, it is sure right that you can combine both, finding an equilibrium that allows doing both without influencing the performance too much. On the other hand when you do cardio with the purpose to improve your cardiac output and overall physical condition, you should probably do it pretty close to the max effort. The same goes for apnea - if you want to improve and adapt your organism to it, you should do it close to the max. I can barely imagine though doing both cardio and apnea with the maximal effort without one having negative impact on the other.
On the other hand, if you rather train for real-life (recreational) diving or spearfishing, where you first have to swim a long distance, then dive, and then swim back again, then indeed such combined training may be a good preparation.
EDIT: somehow I always compare my apnea trainings (especially the static ones) to yoga. And also at yoga, my teachers advised avoiding cardio training, increased physical effort, and food for several hours both before and after the yoga lesson. I find it quite well corresponding to freediving too. However, I am in no way a training expert, so do not take my words for given. Hopefully others will peek in, who understand better the way the body adapts during a training.