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.