Half a Cleric 5 (rounded down) can't cast Silence, and half a Wizard 5 (rounded down) can't cast Ghoul Touch, but that's just a technical detail.

A deeper issue is that it's unrealistic to balance against a very poorly built character like a Wiz 5 / Clr 5.

For an unoptimized game, you'd want to balance against a Wizard 3 / Cleric 3 / Mystic Theurge 4.

For a more usual game, you'd want to balance against a Wizard 1 / Cleric 3 / Mystic Theurge 6.