What is better for health, 2 weekly sessions of 1 hour of cardio or 4 weekly sessions of 30 minutes of cardio? Both at 65-85% of my max heartrate.
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I would work out 2 weekly sessions of 1 hour. You're working your muscles just long enough for a good workout and 30 minutes will do some for your health, but I'd say it's a waste of time and would be easier with your first alternative.
I would follow the anecdotal rule of thumb that greater training regularity with sufficient intervening recovery produces a greater adaptive effect for most types of exercise. I am fairly sure you could find confirmation of this rule in particular cases, asking particular questions about types of adaptation, but a hand-waving argument would be that training signals to the body a need for some extra capacity (greater vascularisation, greater heart volume, etc), but the processes that bring about this increase of capacity don't tend to respond that strongly to volume, that is they tend to saturate (possibly literally in their chemical component) after they are triggered, even if they are triggered 'more'.
This diminishing returns from volume is certainly seen in resistance training. I suspect the picture is a bit more complicated for cardiovascular training, particularly endurance training, but for the periods of time you're talking about I think 4 30 minute session would be preferable. It is certainly my experience that I have seen better results from more regular running over shorter distances.
One other thing to mention is that if you were willing to exploit the fact that your session is shorter to maintain or spike at a higher intensity then the case for the superiority of 4 sessions is much clearer. High intensity training has all sorts of benefits including those of longer steady state sessions (lots of answers on this site bang on about this if you care to search)