mysql multiple column index Cardinality order
Not sure if this answers your question, but I would not bother so much about the individual cardinality of X and Y. For index design I would start with the predicates that the index should assist with. If you have queries like:

where X = ?
where Y = ? and X = ?  


go with (X, Y). If you have queries like:

where Y = ?
where Y = ? and X = ?  


go with (Y, X). If your queries are like:

where X = ?
where Y = ?
where Y = ? and X = ?  


go with (Y, X) and (X, Y). If your queries are like:

where X = ?
where Y = ?


go with (X) and (Y)

I assume here that the cardinality for each column is greater than 1. For cardinality in general I consider the full key cardinality (all involved columns together) to be of more importance than the individual cardinality.