This was suggested but it has a few issues. One, you'd have to specify where each "version" of the sale would go (which slot). Then it'd be complicated to figure out how to individually manage the stock for each one.
What has also been requested, and would make the stocking part easier, is if vendor sales could be stocked directly from Void Storage. Then you just make your two versions of your XP Bottle Sale and you don't really have to worry about stock anymore, just fill up VS. We may look into the VS stock option shortly.