awesome. can you do this with Audacity?
You can align two sources in Audacity. Read the long thread in the sticky at the top of the "Computer Related Help" section of TS.com. I have to caution you that it's not really all that easy, especially if a show is on the longer side. There's math involved, which I've done before and still had things not sync. Dan and I were, for instance, trying to sync 6 stereo mic sources, from mics that were almost in the identical spot in the venue, from two of the same decks (Oade R-44s), and even with all the lining up and everything, the drift still seemed to occur... if you line up one point, another would get more "off".
Those were 2.5 hr shows though. For your normal rock show of 70 or so min, if you line up a cymbal hit and you start the recorders at the same time, you're 95% of the way there probably. Heck, I once did a show where I added in DPA 4061>littlebox>m10 to the 4 channels on my -44. I started both decks at the same time, and I didn't even have to shift the sources at all, as luck would have it.
But anyway, it's not that easy.
Since you have to spend money no matter what you do, I'd second the suggestion of the DR-2D. I haven't used it, but it appears to do what you'd need it to do. You'll still need SBD cables, of course. I personally just use some very long RCAs most of the time and have adapter tips for 1/4" and XLR. Most SBDs I deal with use RCAs, although better ones for bigger venues probably will not.