Very interesting problem.  I will do it so that the outermost dimension of each datavariable is UNLIMITED unless its has been defined already.

This begs the question of whether a preprocessing of the list of datavariables is necessary to determine suitable dimensions.

Example:
Cube1 x(Unlimited), y, z
Cube2 y(unlimited), y(unlimited), z

This requires redefining the "y" dim as unlimited (which we cannot do)

I think this would most likely only be of concern to some situations with commandline tools (may be something to consider in future).