Although the rule works for the Fighter-Cleric, it doesn't look good for the Thief-Cleric
On an XP vs XP comparison:
Thief-Cleric vs Thief:
-Weapons: If we keep the "all the best" rule then it gains nothing, otherwise it loses some weapon access.
-Armor: It should gain more armor, but I would rule that some thieving skills will be hampered.
-Hit Points: I'm using d6+2 as a "best of both", except for very few levels the combo gets the advantage in both median and max values, min values is the reverse.
-Attack Bonus: The single class fights equal or better than the combo, never worst, at lower levels they are more close, at higher levels the difference increase, but not much.
-Backstab: No change.
-Thieving Skills: Due to XP value the combo gets a lower progression, and it can be hampered by armor as stated above (this is assuming the combo can use every skill situationally as a thief could).
-Turning Undead: This is gained.
-Spellcasting and Turning: This are gained.
Its a sturdier but worst at combat thief, maybe with more armor depending the activity performed, with clerical special abilities (but worst than a cleric).
Thief-Cleric vs Cleric:
-Weapons: It should earn more weapons.
-Armor: Gains nothing, could lose based on thieving skills.
-Hit Points: The Cleric gets the upper hand, its only after the combo stops rolling HD that it gets different (using a +2 from the thief).
-Attack Bonus: The cleric has better or equal AB, mostly equal, the better levels are few at first, then become the most part at the higher levels.
-Backstab: This is earned.
-Thieving Skills: These are gained.
-Turning Undead: same at lower rate.
-Spellcasting and Turning: Same at a lower rate.
Combining worst HP with equal or worst AB and the probably use of lower armor to allow thieving skills the combo should refrain from combat compared to a full cleric.
It may work as a tomb raider, using spells to boost their thieving skills and for healing, and maybe using turning undeads as a countermeasure for sneaking into some places