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Eh... Foster's guide is not terribly accurate, having (I think) come out pre-Letters. If I recall correctly it also claims that sailing West granted Frodo immortality which we now know is not the case (and even then, could have guessed that it wouldn't be the case because of the way the events of Numenor played out in the appendices to LotR). Since Celeborn (who apparently sailed at an undistinguished time) also did not sail when Elrond did and we have the additional cases of Sam (probably) and Legolas, there's nothing to say that the sons of Elrond couldn't have sailed sometime in the Fourth Age.
For what it's worth (which is very little) the arguments from a literary standpoint which I've seen fanauthors make when they try to work out which kindred they think Elladan and Elrohir chose usually have them side with the elves: the only two Halfelven who we know sided with mortals had some great good that their choice accomplished. Elros founded the ruling line of Numenor, and Arwen helped to restore the glory of the Dunedain in the Renewed Kingdoms. There appears to be no similar destiny awaiting the sons of Elrond should they choose mortality.
And of course, we can't forget that these are two individuals we're talking about. They don't necessarily have to choose the same way.
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