I say go for it. My cycling club scheduled our winter training camp the same weekend of the big half marathon in town (that I had already registered for). So I’m going to do 2 big days of riding right before a half. We’ll see what happens. I think I may do another half marathon 2 weeks later as well just to see how much faster I could go with more rest.

I've done 5k the day before a half I think four times. I either tempo ran or fun-ran the 5k, then raced the half, or I'd race the 5k, then do the half as part of a long run. One saturday I raced a 5k, then Sunday I ran 7 miles before a half and ran the half at 8:00 pace.

Every time there was one race I took more seriously than the other and that was the one I raced. I wouldn't try to race both.