In the beginning, way back when,there was a plan to develop an interconnection between the first tinyG and a second tinyG board.
Not much interest and that capability was overtaken by other time consumers.
The firmware does decode actions for any four of the 6 axes, X,Y,Z,A,B,C.
I have never heard of commercial Gcode generators that do much, but some folks have built custom three axis rotational machines driven by custom Gcode.
If you are really interested in multiple axis machines, have a look at
https://github.com/synthetos/g2/wiki/9-Axis-UVW-Operation.
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