Short answer. They divide the substituted share in proportion to their own shares in the original institution. Article 861 of the Civil Code provides that if there is more than one substitute, each substitute shall have the same share in the substitution as they have in the institution.
What the law says
If heirs instituted in unequal shares should be reciprocally substituted, the substitute shall acquire the share of the heir who dies, renounces, or is incapacitated, unless it clearly appears that the intention of the testator was otherwise. If there are more than one substitute, they shall have the same share in the substitution as in the institution.
Civil Code, Article 861 — Reciprocal Substitution. Read the full provision →
Reciprocal substitution and unequal shares
Article 861 of the Civil Code governs the situation where a testator designates several heirs as each other's substitutes. The first rule addresses what happens when the heirs were instituted in unequal shares: If heirs instituted in unequal shares should be reciprocally substituted, the substitute shall acquire the share of the heir who dies, renounces, or is incapacitated, unless it clearly appears that the intention of the testator was otherwise. The default is that the substitute steps into the shoes of the heir who cannot or will not take — receiving that heir's full designated share — unless the will shows a different intent.
How the substituted share is divided among multiple substitutes
The second rule in Article 861 addresses your specific question: If there are more than one substitute, they shall have the same share in the substitution as in the institution. This means the proportion in which the substitutes divide the substituted share mirrors the proportion in which they were originally instituted as heirs. If Substitute A was given twice the original share of Substitute B, then Substitute A receives twice the portion of the substituted share that Substitute B receives. The key is proportionality — the original institution ratios carry over into the substitution.
A concrete example of how this works
Suppose a testator institutes three heirs — A with a one-half share, B with a one-fourth share, and C with a one-fourth share — and designates them as each other's reciprocal substitutes. If A dies before the testator, A's one-half share goes to B and C as substitutes. Under Article 861, B and C divide that half in the same proportion as their original institution: B had twice as much as C in the original institution (one-fourth to C's one-fourth), so in this case they would split equally. If instead B had been given a three-fourths of the remainder and C a one-fourth, they would divide A's share in that same three-to-one ratio.
When the testator's intent overrides the default
Article 861 applies as the default rule — but the article also preserves the testator's authority to set a different arrangement. If the will clearly appears to express that the substituted share should be divided differently among the substitutes, that expressed intent governs instead of the proportional rule. This requirement of clarity is important: courts will not infer a different distribution from ambiguous language or silence. The proportional rule is the fallback precisely because it represents the most natural extension of the testator's original allocation. Only an unambiguous expression of a different intent in the will itself will displace it.