I've since found lots of old posts on the subject, and I assume that JRiver being able to hardware control multiple devices to sync off a master clock is a no go.
The issue isn't in JRiver's ability to control, it's in the hardware's ability to accept an external clock signal. I've never seen a Receiver that had that capability, and many commercial sound devices can't either. If you want to use 14 channels, you'll need to either buy one interface that has fourteen outputs, or buy two interfaces that support syncing enough channels.
SPDIF would not solve your problem as SPDIF can only send two channels uncompressed so you couldn't even daisy chain a solution that way unless you had an eight analog channel out device with 3 SPDIF outputs. Two Steinberg UR824's would probably work because the ADAT inter-link can send 8 channels with a clock, but really you're probably better off buying one device with enough analog output channels.
Alternatively, you could buy two 2x6 mini-DSP devices and place them in the signal path for your tri-amped speakers, but that doesn't solve your sub issue.
Bottom line: you need interfaces that share a clock signal to have a reliable setup, which means:
1) One interface with enough analog outputs
2) Multiple interfaces that collectively have enough analog outputs that can accept a word-clock input separate from the audio signal (expensive, but they exist) and a word-clock source.
3) Multiple interfaces that collectively have enough analog outputs, one of which can send a multichannel ADAT output and one of which can accept a multichannel ADAT input (ADAT can usually send 8 or more channels with the clock)
4) One interface with several SPDIF or AES/EBU outputs, and devices that have SPDIF or AES/EBU inputs that collectively have enough analog outputs. A conventional SPDIF output is typically a stereo pair with a clock. You'd need 7 such outputs. A Lynx AES 16e would work as a clock source, but you'd need actual DACs with AES/EBU inputs to take advantage of it obviously.
5) Some combination of the above.
USB and HDMI don't typically have an embedded clock or most devices receiving such an input ignore the clock (same outcome).