You can already do damage while healing someone with a macro like this.

/action "Stone" <t>
/action "Stone" <tt>

You can do the reverse and heal someone while a boss is your target and a player is your focus target.

/action "Cure" <t>
/action "Cure" <f>
/action "Cure" <0>

You could also make these do the last target with <lt> or you can make it heal the main tank with <tt>.

It seems pointless for this to be presented as a unique feature of Sage.