The timing does not change due to using single or dual points. It is the same. However the dwell is a function of both sets of points. Dwell is the time the points stay closed and since both sets are wired in parallel, the dwell time is increased. This allows the magnetic field in the coil to saturate at higher rpm than would be possible with only a single points set. The end result is the coil voltage (spark) doesn't drop off as fast at high rpm. To adjust the points, one "blocks off" one set and sets the dwell of the remaining set just as if they were single points. Dwell of a single set is set to about 28 degrees. Then the adjusted set is "blocked off" and the remaining set is adjusted for the same dwell angle, about 28 degrees. The "blocker" is then removed and the dwell is rechecked. The combined dwell should be about 33 degrees. After dwell is satisfactory, timing is set in the traditional fashion, typically 6 degrees BTDC with vaccuum advance plugged and the engine run not greater than 600 rpm.
If the engine won't idle down to 600 rpm then set the timing at 1 degree more per 100 rpm. Example if the engine runs at 1000 rpm lowest, then set the timing 4 degrees more advanced. (1000-600=400), 400/100=4 degrees more advance. Why 1 degree per 100 rpm? Total advance for most cars is 24 degrees mechanical. This occurs at 3000 rpm. 3000-600=2400rpm difference. 2400rpm/24degrees=100rpm/degree. If the car has no vaccuum advance, 1.5 degrees per 100 rpm is more appropriate.