Sim, eles fazem
O Feiticeiro Mágico das Sombras's Força do Túmulo O recurso (XGtE, p. 51) diz:
When damage reduces you to 0 hit points, you can make a Charisma saving throw (DC 5 + the damage taken). On a success, you instead drop to 1 hit point. You can’t use this feature if you are reduced to 0 hit points by radiant damage or by a critical hit.
After the saving throw succeeds, you can’t use this feature again until you finish a long rest.
O Traço de resistência implacável do Half-Orc lê-se:
When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest.
Nada sobre essas habilidades proíbe explicitamente o uso da outra. Se você falhar no salvamento, continuaria com o 0 HP, que satisfaz a condição para o uso do Relentless Endurance. Como resultado, você pode usar as duas habilidades se Força do Túmulo falhar.
Isso corresponde à intenção das regras, conforme confirmado em um tweet não oficial do principal designer de regras Jeremy Crawford:
are features like barbarian relentless rage and shadow sorcerer strength of the grave intended to stack (if you fail one saving throw, try the other) or is it a choose one in an instant scenario?
Things in the game don't shut each other off unless a rule explicitly says otherwise.