The only real way to do this is with a modifier; however, you need ready access to both abilities. I advise that you don't put those on the same key as holding a mod the entire time a mob is below 35% would be annoying.
You can put finishers together without much issue, though. You'll probably know what you're going to use 1-2 cycles in advance. I use these:
/cast [mod] Recuperate; Eviscerate
Eviscerate turns into Envenom when Assassination, so these macros work for all specs.
/cast [mod] Slice and Dice; Rupture