Weekly alternating cronjob (with step values)
Solution 1:
Since you know that one and only one Saturday will occur in the first 7 days of any month, you can follow that pattern and use the day-of-month field to keep each job separate:
30 3 1-7,15-21 * 6 script1
30 3 8-14,22-28 * 6 script2
Thus script1 will always catch the first and third saturdays and script2 will always catch the second and fourth.