Rayl, you could accomplish this with…
/castsequence Rip, Regrowth
…but it might make more sense to instead make a macro like the following:
/cast [harm] Rip, [@mouseover, exists, nodead, help] Regrowth
That will always use Rip when your target is hostile, and always use Regrowth when your target is friendly (or when you mouseover a friendly target - such as yourself). It isn’t precisely what you’re looking for, but it does combine 2 spells that do not overlap usage into a single button, and it is somewhat more versatile because Regrowth or Rip will never be locked behind a /castsequence order, where you might not have the one up that you want at a critical time.
If you feel like going down a pretty deep rabbit hole with macros, check this out: Macros: Essential Information