Yeah, that worked! Ultimately, I'm getting 20k points in 840us, so an equivalent of 24Msps. See below for more analysis on a signal at around 1.9 MHz:

See the full log at https://github.com/kelu124/echomods/blob/master/elmo/data/20170713-TwoADCs.ipynb -- thanks to macrofab for their perfect PCBA role =)