There are two related but distinct applications of these phrase.
Their commonality is the sense of losing something that cannot be recovered.
Their differences:
- The first unpardonable sin--willful disobedience to God--is almost ubiquitous. Almost no one has committed the second unpardonable sin, because almost no one has achieved the state of union with God required to commit it.
- The first unpardonable sin causes you to permanently lose what can only be gained in life--the highest degree of glory with God after life. The second unpardonable sin causes you to permanently lose all glory with God here and hereafter.
- The first unpardonable sin can be ceased by your repentance during mortal life. In this sense, it is not unpardonable in the sense that it cannot be repented of, but in the sense that while you are in it, you cannot be forgiven for it. The blood of Jesus can wash you from this sin if you forsake it. The second unpardonable sin cannot be repented of. Once committed, you are condemned to suffer the penalty of the sin.
Other people have had at least some knowledge of these ideas. Joseph Smith described them in his sermons, though I do not believe he reached the necessary resolution to understand or describe the distinctions I've laid out here.
In "Repentance," I intentionally omitted reference to the second, graver application of this phrase. Why? Because I have yet to meet another person in the mortal sphere who is capable of committing it. We ought to focus our limited energy on the ladder rung before us instead of spending it chasing after mysteries that do not yet pertain to us.
You will never know God sufficiently to commit this second unpardonable sin as long as you are committing the first unpardonable sin. Until you attain a sinless state, the only thing that matters is achieving a sinless state.