I'd say switch the two around.

/micon "Cure"
/ac "Cure" <t>
/ac "Cure" <tt>

This way if you have a player targeted it will heal them, but if you have a mob targeted it should skip the first command and fire the 2nd.
Its not the way I have heals set as I use the focus target (usually tank) and mouse over for other heals.. I'll post those commands if you're interested.