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I think there are several important factors to consider.<br><br>
First, time spent running. If you are running more than 10 hrs per week you are probably a candidate to do doubles. I have found that I can only go higher than that time consistently when I am doing doubles. For me, I approach that level somewhere around 60 miles per week. Faster runners would be able to do more miles before splitting.<br><br>
Second, the race distance you are training for. Longer distances require longer training runs, and I would be loathe to split runs if I was training for a HM or a marathon, but I think people training for 3000 to 5000 meters might benefit from doubles because they require a bit more intensity.<br><br>
Third, lifestyle. Some people just don't have the time to do long singles, and there is no doubt in my mind that short doubles are better than short singles anytime.<br><br>
Finally, I would never double up on a long run. In my opinion they have a very specific purpose that is defeated if you split them. They should ideally be in the 2 hr to 2.5 hr range, and doing two by 1 hr runs just won't give you the stimulation your legs need to adapt.

 

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Alex, My daughter was a HS senior last year and I set up a program for her with a 2 hr run and a 1.5 hr run each week. She didn't do them all because she had other things going on in her life, like piano, volley ball, boyfriend, etc, but she was trained well enough to run in the provincial HS track finals this spring. Ideally I think you should run up to 2 hrs for your long run. Mid week you should run 90 min. All of the rest should be shorter. Note that I said "ideally" because if you are not used to a 2 hr run you should take some time to build towards that level. 5k XC demands power as well as endurance, so you should probably think about some Lydiardesque hill sessions on one or two days per week.