Ok - I don't see why not only one miniDIGI could be used. I2S is only for very short cable distance, so if the miniDSP/MiniDIGI combo are not stacked, then you have to use SPDIF between the MiniDSP's and therefore two MiniDIGI boards.
But I will recommend to ask the development teamt them self. They usually answere within a couple of days.
You can order the custom jumper cables by email. But the stack was never designed with 1 x digi and 2 x DSP, so you will have stereo with digital in but when you switch to analog only one channel will work. I don't know of any way to fix this. You may be better off buying the 2 x 8.
I was a bit surprised that MiniDSP never mentioned the fact that you cant switch analog when they sold me the extra "miniDSP + miniDIGI + Custom Jumper Cables". It took about a month of testing and emails with them to let me know.