Went through realmshelps for any "Death Attack"
that also has stacking language (or similar).
Very few have that, usually with more specifics.
Most of them have the same wording as the
Assassin PrC as if nothing else would stack.
Monk Of The Long Death , in Players Guide To Faerun
If the monk of the long death also has the death attack ability from another class (such as assassin), levels of that class stack with his monk of the long death levels for the purpose of determining the save DC.
Imaskari Vengeance Taker , in Underdark
A vengeance taker with levels in the assassin prestige class may add her assassin and vengeance taker class levels together for the purpose of calculating the save DC for her vengeance taker death attack.
Thayan Slaver , in Unapproachable East
Enervating Attack: As the assassin death attack ability, except that Thayan slavers have only the option of paralyzing their victims. If a Thayan slaver has levels in the assassin prestige class, he may add his assassin and Thayan slaver class levels together for purposes of calculating the DC of his enervating attack. Slaver class levels do not add to the DC of the assassin's death attack.
Perfect Wight , in Epic ... though more of a feat add on type
Improved Legerdemain (Su): A perfect wight can perform the following class skills at a range of 30 feet: Disable Device, Open Lock, Pick Pocket, and Search. If desired, the perfect wight can take 10 on the check. Any object manipulated during the skill check must weigh 100 pounds or less. Alternatively, the perfect wight can use improved legerdemain to make one melee sneak attack against any creature within 30 feet. The perfect wight executes the sneak attack (or death attack, if applicable) as if attacking from a flanking position. If the attack is successful, the victim is dealt the appropriate sneak attack damage despite the fact that the perfect wight and his or her weapon do not physically cross the intervening distance. A perfect wight can use improved legerdemain once per day at 2nd level, plus one additional time per day every five levels thereafter.
... and ...
Black Flame Zealot , in Complete Divine
If a black flame zealot has this class feature from another class (such as assassin), he may add black flame zealot class levels together with the class levels of such other classes to calculate the save DC of his death attack.