even though the other rabbit is as much for you as for Charlie, it's SO much less work to be able to keep them together and they get twice as much time with you that way, so it's far better to find someone for him to bond with.
if charlie turns out to be a girl, I would go for a boy... yes, some of us here have F/F bonded pairs... but we've all got stories of those bonds very suddenly and inexplicably being broken or damaged. it's possible to mend such a bond (a couple hours in neutral territory has always worked for me), but it's frustrating, it's a hassle, and it's scary to see two girls who are normally very lovey-dovey suddenly attacking each other or running in fear from their bond-mate.