Its a bit hard to tell from their specs page without a bit of interpolation but will one of each line up side by side? Seems the D688 isnt available anywhere but ebay. Would be nice if they lined up so as one block would bolt spanning between both if needed since I have a 16" long part.
Didn't get any answer here so I emailed Kurt directly for an answer. The short is if you bolt them both on the table parts will span both vice's and line up properly. The only downside is the DX6 is longer by about 0.4" on the backside. With the D688 the vice just rubs the z-axis way cover just slightly at full Y-travel and the DX6 will likely hit it.
I stalked ebay and found a new unopened D688 for under $600 shipped so went that route.