If you have very fast edges in your digital signals you could see crosstalk from one wire to the next one over. The easiest fix for this is to not have very fast edges -- which can usually be achieved by putting resistors in series with each data line to slow down the edge rate. You'll have to tell us more about the "digital data streams" to determine whether or not this will work.
You also mentioned that the data lines will have 20mA of current flowing -- is that because they are optically isolated? If so, you already have a pretty good method of noise immunity that is not going to depend so much on edge rates or how close the signals are.
Without any resistors or optoisolators, I would begin by separating data lines with ground lines, for example: data<-->ground<-->data<-->9V<-->data<-->ground, etc.
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