Short answer. Yes. The second heir acquires a right to the succession from the moment of the testator's death, even if he dies before the fiduciary and before the property is delivered. Because that right vested in him, it passes to his own heirs, who take the property when the time to transmit it arrives.
What the law says
The second heir shall acquire a right to the succession from the time of the testator's death, even though he should die before the fiduciary. The right of the second heir shall pass to his heirs.
Civil Code, Article 866 — The Second Heir's Right Vests at Death. Read the full provision →
The second heir's right vests at the testator's death
Article 866 fixes when the second heir's entitlement arises: the second heir shall acquire a right to the succession from the time of the testator's death, even though he should die before the fiduciary. The crucial point is that the right does not wait for actual delivery. It vests in the second heir the instant the testator dies, while the fiduciary is still holding the property. So although the second heir may never physically receive the inheritance in his lifetime, the right to receive it was already his from the outset, not a mere hope contingent on his outliving the first heir.
So it passes to his own heirs
Because the right had already vested, it does not die with the second heir. The article continues: the right of the second heir shall pass to his heirs. Having become part of what the second heir owned, the entitlement is transmitted through his estate to those who inherit from him. So the property does not revert to the fiduciary, nor fall to the testator's other heirs, simply because the second heir predeceased the fiduciary. When the moment to transmit arrives, the fiduciary delivers to the second heir's successors, who stand in his place.
Why vesting early matters
The rule reflects that a fideicommissary substitution creates two successive heirs both called by the testator from the same death, not a chain in which each must survive to a later date. The second heir is a true heir of the testator, taking his right at the testator's death, with only the enjoyment postponed until the fiduciary's obligation to deliver falls due. Treating the right as vested from that first moment is what allows it to survive the second heir and reach his own successors, honouring the testator's plan even across an intervening death.
What this means in practice
If the second heir has died before the property is handed over, his heirs should look to whether a valid fideicommissary substitution was set up and when the testator meant the transmission to occur. The fiduciary's duty to preserve and deliver continues, now running in favour of the second heir's successors. Gather the will and the record of the arrangement, since the terms fix the timing of delivery and confirm that the right vested and passed. A lawyer can trace the entitlement from the testator, through the second heir, to those now standing to receive.