Your best bet is to definitely use a mixer, since the br-8 has only 2 inputs. However, do not use the vocal mic jack. This jack has a pre amp in it which is not necessary because your mixer should already have the mic pres taken care of. Instead use the rca input to record the stereo output of the mixer to two tracks.
Also, when I was recording with a br-8, I didn't usually go for a stereo drum sound. When limited to just two tracks of drums I found it much more useful to put the snare and toms on one track and the bass drum/cymbals on another, and then just keep it in mono. This gives you much more control over the mix and eq in the end and allows you to put some nice reverb on the snare and toms without washing out the cymals and bass drum. IMOA the reverb on the br8 makes cymbals sound like absolute sh*& so I try to keep it away from them as much as possible.
Run a search for info on how to mic the drum set. If you choose to go mono, then you don't really need both overhead mics.
