Double Anal is the New Anal
As anal sex becomes increasingly popular and normal, it is shedding a lot of that stigma and perception as something done occasionally as a special treat but not a regular activity. For many now, anal is just as normal and common as vaginal, if not more common or completely replacing it.
Just as oral sex was once a relatively uncommon thing in our culture, only done on special occasions or by certain people, but has now become a standard and extremely frequent part of a healthy sexual repertoire, so has anal become the same.
And with the rapid increase in popularity of anal sex, there are a growing number of women for whom anal sex comes quite easily and who are looking for something to fill the role that anal once did, as something more intense and challenging which they may not do every time, but which is fun to do on occasion. For those who are in the anal only lifestyle, the old fashioned form of double penetration involving a combination of vaginal + anal penetration is naturally out of the question, but double anal penetration is a viable option.
Growing in popularity in its own right in recent years, perhaps in part due to increased exposure through pornography, double anal is being adopted as a practice by real world women and couples, whether with toys, multiple partners, or a combination of partners and toys.