Well... You sorta can I guess, but I doubt it will work like you want it to.
As far as I know:
Code:
/cast [target=mouseover,exists] Healing Surge
/cast Stormstrike
should cast Healing Surge on to a mouseover target, when one exists i.e. you are hovering over one. If one does not exists, the condition is not met and it it drops down to casting Stormstrike.
The problem is, that there is no way of telling it to only cast when you are hovering over unit frames. The same functionality works when hovering over a friendly target in the game world, so you'd end up accidentally casting it on people.
I'd recommend just having it on a separate bind on click. I'm sure some ISBoxer trickery could be used to create a bunch of repeater regions or buttons that trigger specific mapped keys when your mouse enters in to the region, but this seems like a very overkill, and unflexible way of doing something that could be done with a simple click.
For example you could create a mouseover bind/click for setting focus, and then macro your abilities like this:
Code:
/cast [target=focus,exists] Healing Surge
/cast Stormstrike
This is similar to the previous setup. It casts Healing Surge on your focus if one exists. If not, then it goes to Stormstrike. Of course what this means is that as long as there is a focus, Stormstrike is not cast ever. This obviously means you also need to be able to drop focus when you want to stop using Healing Surge.
Or perhaps you could to separate the 2 lines of the macro in to different actions in the same mapped key. So for example:
Mapped Key (Stormstrike)Step 1
- Macro that sends "/cast [target=focus,exists] Healing Surge"
- Macro that sends "/cast Stormstrike"
It's not perfect, but I think it would be functional.