The DR2d is primarily designed a stereo recorder and like most pocket sized decks has two built-in mics and separate mic and line-inputs. It's unique in that it has a feature called dual record, targeted primarily at making a second 'safety' stereo recording at a lower level, in case the primary recoring clips. In that configuration it is two channels in, but writes two seperate stereo files. Alternately it can be set for two seperate stereo inputs, comprised of either the 2 internal mics + a stereo external input, or 2 external stereo inputs using both mic and line-inputs simultaneously. You can choose to mix those two stereo inputs internally and write a single stereo file, or write two seperate stereo files. That configuration is primarily targeted at mics plus a soundboard feed.
That last configuration is how I use it- except instead of adding a soundboard feed, I'm using four external mics into twin CA-UGLYs, one feeding the mic-input, the other feeding the line-input. I end up with two stereo files, providing 4 synced channels.
I previously used two R-09s with the same mics and preamps. The DR2d allows me to eliminate one recorder and have the resulting files synced perfectly from the start. The ability to lock-down and start with the remote is a very nice benefit, all the more so in my case because not only do I not need to start and stop two recorders anymore, I don't even need to take it out of my pocket to do so. The external Energizer battery pack is key in that I don't need to worry at all about leaving the recorder on for long periods before or between sets and running out of juice. I may be able to power both preamps from the same battery pack, but haven't tried that yet. Doing so wouldn't really reduce size much, but would make for an easy way of managing and checking only one battery with more than enough reserve power.