I myself have found it easier to do workouts that i can handle heavier weights (ie compound workouts which HST relies on) to avoid this problem. But you could also do 2 days of 5 lbs, 2 days of 10 lbs and then 2 days of 15lbs. The real problem you are going to run into though is zig-zagging bad. Since you are supposed to ever increase your weight, if your 15rm is 15 then i would imagine your 10rm could not be much more than 20-25lbs. This means on your first 2 weeks (15's) the weight will go from 5-15lbs, and your next 2 weeks (10's) would go from 10-20 lbs. Meaning the first 4 workouts (2 @ 10 lbs and 2 @ 15 lbs) would be overlapping your previous 2 week mini cycle, make sense? This is why it is easier to do workouts where you can handle higher weight.