The stock-standard answer to this age-old question is "the promise and the oath" as per Ellicott, Barnes, Poole, Meyer, Expositor's Greek, Cambridge, Bengel's Gnomen, Vincent, etc.
Let us examine Heb 6:18 more closely, but to do this we need to back up to v13 which I quote below (BSB):
When God made His promise to Abraham, since He had no one greater
to swear by, He swore by Himself, saying, “I will surely bless you and
multiply your descendants.” And so Abraham, after waiting patiently,
obtained the promise.
Men swear by someone greater than themselves, and their oath
serves as a confirmation to end all argument. So when God wanted
to make the unchanging nature of His purpose very clear to the heirs
of the promise, He guaranteed it with an oath. Thus by two
unchangeable things in which it is impossible for God to lie, we who
have fled to take hold of the hope set before us may be strongly
Note that the oath to confirm the promise has validity because God does not change and His purpose does not change. Thus, the promise is confirmed by the oath which cannot change because that which it is sworn by is God Himself.