Alright. I'm getting ready to chokeslam this device.
ok, not really. But I would like to know the answer.

I know when setting up a stateful failover between ASAs, you can use the same the same line for the failover and stateful links, however, when I try and turn stateful on (failover link stateful_link x.x.x.x x.x.x.x standby x.x.x.x), I get an error saying it's already in use as the failover link! What the heck? Cisco told me they can share the link.

Tell it to me straight, boys and girls. Did they lie to me?