If you want a one button macro I think you can do something like:

/ac “Heart of Stone” <2>
/ac “Heart of Stone” <me>

The me may be unnecessary but in this instance it should default to target 2 if available (which if default party list order is your cotank) and if there is no 2 it will target yourself. That being said this macro will not be useable if you intend to use it on yourself in a party setting.