Yes, quite a few people do. The "polymono" configuration is not uncommon in poly circles. I am the "poly" part in a polymono relationship, so am fairly familiar with the issues and understand how it can be a bit daunting.
I'm not sure if you are familiar with the writings of Franklin Veaux but I think he writes some excellent articles on polyamory in general. In this case he has one called "Polyamory for the Monogamous" and it can be found at http://www.xeromag.com/fvmonopoly_existing.html
He lays out some very good "points to ponder" about the subject, in my opinion. Hope this helps you too!