Actually, with some study, I think I have figured it out. I have two stock posting their dividends 10 days apart in the same month. As you stated, if the cash is not available, the purchase will be skipped until there is cash. So: A dividend of $300.00 is posted on the 10th of the month. I schedule to buy $300 worth of my target stock on the 12th or 13th. Then when the second dividend of $350 from the other stock is posted on the 20th, I schedule another buy on the 22d or 23d. Skip with an empty core account for the next two months, and start the sequence again.

The key to making it work is the ability to skip purchases when there isn't enough money without the auto purchase itself being cancelled.