Celsus, Digest, Book XVI. “Let Titius and Seius, or the survivor of either of them, be my heir.” I think that if both of them survive, both will be heirs, but if one of them should die, the survivor will be heir to the entire estate:

Celsus, Digest, Book XVI. The Senate also decided this question where a legacy was bequeathed in the same way.

Celsus, Digest, Book XVI. A man who is free, but who is serving you as a slave, having been appointed an heir, enters upon the estate by your order. Trebatius says that he is the heir, but Labeo maintains that he is not, if he acted through necessity, and not, on the contrary, because he intended to bind himself. 1If anyone should appoint an heir as follows: “Let Titius be my heir to the portion in which he is a partner with me in the lease of the salt-pits,” certain authorities hold that if this statement had been made by the testator after the entire property had been divided, even though Titius was a partner to a very large extent, the appointee would not be the heir; but if there was a certain share which had not been bequeathed, he would be the heir to it. This opinion is absurd and incorrect, for what prevents the testator from legally making Titius his heir for the fourth part, which perhaps was the amount in which he was interested as a partner, after the entire property had been disposed of under the ordinary division? 2“Let Titius be my heir, and let Seius and Mævius also be my heirs.” It is true, as is held by Proculus, that the estate should be divided into two portions, one of which should be given to the two heirs who were appointed together. 3Where one of several heirs who has not been appointed conjointly with anyone else declines to take under the will, his share will accrue to all the others in proportion to their hereditary shares; and it does not make any difference whether any of them was appointed in the first place, or is substituted for someone else. 4Where a person appointed heir was a Roman citizen at the time that the will was executed, and was afterwards interdicted from water and fire, he will be the heir if he should return between the time of his sentence and that of the death of the testator, or if he was appointed an heir under a certain condition, and returns at the time that the condition was fulfilled. The same rule also applies to legacies, and the prætorian possession of estates. 5“Let Titius be my heir to half of my estate, Seius to a quarter, and Titius to the other quarter if he ascends to the Capitol.” If he conducts himself as heir before he ascends to the Capitol, he will be entitled to half of the estate; if he should do so afterwards, he will be heir to a quarter of the same; for it will not be necessary for him to signify his acceptance, since he is already an heir. 6Where the following is stated in a will: “Let Titius be my heir to a third part of my estate, and Mævius be my heir to another third, and let Titius be my heir to the remaining third, if a ship should arrive from Asia within three months.” Let us see whether Titius will not immediately become the heir to half of my estate, for two heirs have been appointed. Titius will either be an heir to one-half of it, or to two-thirds, so that a sixth of the estate will be in abeyance, and if the condition should be fulfilled, Titius will be the heir to two-thirds of the estate, but if it should not be fulfilled, the sixth will accrue to Mævius. If, however, Titius should die before the condition is fulfilled, and it should be fulfilled afterwards, the sixth of the estate which remained in abeyance will not accrue to the heir of Titius, but to Mævius; for Titius died when it was still doubtful as to whether he or Mævius would be entitled to the said sixth, since it could not be understood to have been given to him who was no longer in existence at the time it should have been allotted. 7If Attius should appoint Titius, Mævius, and Seius heirs to equal portions of his estate, and, in the meantime, Titius was the only one who accepted, and he appointed Seius his heir, Seius would enter upon the estate of Titius, and could either accept or decline that of Attius; but before he accepted or rejected the estate of Attius, he would still be the heir to half of it. If Seius should enter upon the estate of Attius, Titius would only be the heir to one-third of the same, and through inheritance only a third of the estate of Titius would come into the hands of Seius, but he would be entitled to another third by virtue of his appointment. But what if Titius and Seius, having been appointed heirs of Attius, Titius should enter upon the estate, and Seius should become the heir of Titius, could he, or could he not refuse the estate of Attius, or would he necessarily be the heir to the entire estate? As no one else was appointed but the person who was already the heir to a certain portion of the estate, it is just the same as if he had been appointed sole heir by Titius.

Celsus, Digest, Book XVI. A slave belonging to another can be appointed an heir, “When he shall become free”; but a slave belonging to the testator cannot be appointed in this manner.

Celsus, Digest, Book XVI. A man for whom his own slave was substituted ordered him to enter upon the estate. If he did this for the purpose of avoiding payment of the legacies, he shall pay them all, both for the reason that he is the heir, and because having relinquished his rights under the will he has possession of the estate by virtue of the substitution, with the exception of the portion reserved by the Falcidian Law.

Ad Dig. 31,16Windscheid: Lehrbuch des Pandektenrechts, 7. Aufl. 1891, Bd. I, § 156, Note 1; Bd. III, § 625, Note 11.Paulus, Digest, Book VI. If a legacy is bequeathed to either Titius or Seius, “Whichever one my heir may prefer,” the heir, by giving the legacy to one of them, is released from liability to both. If he gives the legacy to neither, both can demand it of him, just as if the property had been bequeathed to one alone; for as two creditors can be created by a stipulation, so two legatees can be created by a will.

The Same, Digest, Book XVI. Where anyone leaves his wife pregnant, he is not considered to have died without children.