I know this is already answered, but could not find it.

How does Open the third eye and Third Eye combo works?:

You cast the first, maximum sanity goes up, do you gain a sanity also?

After this, if you refresh the Open the third eye, and your maximum stamina get backs to normal, do you get back 2 staminas?

Thanks!

No, as a general rule you never gain a sanity or stamina when your maximum goes up. Whenever you cast (or release) this spell, you suffer a net loss, or at best, your current sanity/stamina do not change.

In other words: a changing maximum can only lower a stat. It can't raise them.