Warlock macro help

I need some help figuring out a macro. I'm a warlock, so I have a ton of macros revolving around holding alt to switch to a different spell. For Corruption and Seed of Corruption I do the same. This creates an interesting problem. With Seed of Corruption I want to spam it on as many targets as possible, but with normal Corruption I do not. My current macro is this:

/cast [mod:alt] Seed of Corruption; Corruption
#showtooltip
#show

I want it to switch to a new target just like hitting tab (/targetenemy) , but ONLY when alt is held down (when SoC is cast). I have been toying around for days and have not been able to figure it out so I am not sure if it is entirely possible. Can somebody please help me out with this?

Theres no way to automatically change target after you cast a spell.
But you can do:

#showtooltip
/cast [mod:alt] Seed of Corruption; Corruption
/stopmacro [nomod:alt]
/targetenemy

this will change your target after you attempt to use Seed of Corruption