Freebooksy is just starting this feature, but they're charging a lot, $200.
If you have a first in series free, and a second at 99 cents, here is how I would do it:
Book ads for both on the same day with butterfly, bknights, bestebookreaderlovers, genrepulse, and then in ads for the first in series with other places mention that book two is on sale. ReadCheaply, ManyBooks.Net, ENT. You'd get the exposure of the Freebooksy ad at a fraction of the cost.
You could consider creating a box-set/bundle of your first three novels, to gain entry to the promo sites that only do singletons. You could even offer it as a "limited time only" publication and take it off the shelf later, if you thought it was cannibalizing your single sales.
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