How to use greater visage of the deity to be both half-fiend and half-celestial?

[Mister Freeze]

By strict RAW, the spell only affects evil clerics (granting the half-fiend template) and good clerics (half-celestial template instead) when normally cast.  If you were to use the spell from a magic item, you could UMD it to benefit from it both ways, UMDing each use of a magic item as a different alignment to become both a half-fiend and half-celestial.  I don't see why this wouldn't work, unless you also believe spells like energy immunity don't stack with each other when different castings use different elements.  What I'm looking for is a way for a wizard incantatrix who has this spell via arcane disciple to reliably have both templates persisted, avoiding custom magic items.

[BearsAreBrown]

UMD would not work because it is not a limitation of the Magic Device but a limitation of the spell. Same reason you can't UMD a Wand of Cure Light to heal over 8+5.

[zook1shoe]

Worship the five-aspect deity in Serpent Kingdoms?

[FlaminCows]

You could count as both good and evil by having the appropriate alignment subtype. For example, a good-aligned cleric with the Evil subtype.

"Most creatures that have this subtype also have evil alignments; however, if their alignments change, they still retain the subtype. Any effect that depends on alignment affects a creature with this subtype as if the creature has an evil alignment, no matter what its alignment actually is. The creature also suffers effects according to its actual alignment."

[Bauglir]

You'll also need to dip Cleric, RAW.

[Rebel7284]

Holy Transformation [lesser] gives you the good subtype.
Infernal Transformation [lesser] gives you the evil subtype.

Notably, the 7th level Holy Transformation also gives you the archon type, useful for all those wonderful spells in BoED.

[brujon]

Along with all the great suggestions that were already put forth here, there's also a multitude of cursed items that make you count as opposite alignment as long as you're using them/holding them, so equipping them, casting, unequipping and casting again could work.

[ksbsnowowl]

Couldn't a Good Hellbred (FC2) Cleric count as both good and evil for this purpose?

[Tr011]

You could count as both good and evil by having the appropriate alignment subtype. For example, a good-aligned cleric with the Evil subtype.

"Most creatures that have this subtype also have evil alignments; however, if their alignments change, they still retain the subtype. Any effect that depends on alignment affects a creature with this subtype as if the creature has an evil alignment, no matter what its alignment actually is. The creature also suffers effects according to its actual alignment."

Assuming you are LG with the evil subtype, you count as evil for the spell. You would have to remove the evil subtype before you cast the spell again, so your true alignment (LG) is gonna be used. PAO can do this. But it's still not a way you can use evil-spells in an actual game without being hellbred.

Couldn't a Good Hellbred (FC2) Cleric count as both good and evil for this purpose?

Don't think so. A Hellbred only can use the evil spells in the first place without being evil himself.

[Kethrian]

You could count as both good and evil by having the appropriate alignment subtype. For example, a good-aligned cleric with the Evil subtype.

"Most creatures that have this subtype also have evil alignments; however, if their alignments change, they still retain the subtype. Any effect that depends on alignment affects a creature with this subtype as if the creature has an evil alignment, no matter what its alignment actually is. The creature also suffers effects according to its actual alignment."

Assuming you are LG with the evil subtype, you count as evil for the spell. You would have to remove the evil subtype before you cast the spell again, so your true alignment (LG) is gonna be used. PAO can do this. But it's still not a way you can use evil-spells in an actual game without being hellbred.

Bolded for emphasis.  It counts as evil due to the subtype, but also counts as good due to its alignment.  It "suffers" effects as both.