The image is not mine but his, looks like an Eagle PCB layout (perhaps).
KO7M mentioned this is still a prototype that the SMA connectors kinda foul with the USB connector from the Arduino Uno underneath... I guess moving them down a bit (on the image) would work but too much and then Arduino power barrel connector socket then might foul with BFO SMA... I wonder if it might better if the SMA connectors were mounted 90 degrees to the PCB, the ability to stack the boards might be effected (if another were to be added).
The pins would have to be cut very close to the underside of the board (unless SMT versions used). I expect LPF would need to be fitted to the Si570 outputs anyway and these might be better off board anyway.
I've seen a similar usage suggested recently using those little AD9850/AD9851 boards from ebay/china... one provides the VFO the other the BFO frequency.
These boards contain an LPF (a poor one) the little row of SMT devices between the blue output pot and the 1/2 size clock can are an LPF.... It is better to just remove it and use an external LPF.
This is just a single DDS from Kanga and it is mentioned here http://m0xpd.blogspot.co.uk/2013/08/kanga-m0xpd-dds-shield.html
I picked one up at G-QRP Convention at Rishworth and looked over a dual DDS version (not a shield yet) that was being used with a clone of Drew Diamonds TCF Transceiver.