Regrettably, the scriptio continua (‘continuous writing’, that is, the omitting into the text the punctuation marks, or equivalent symbols of start/end of a term/phrase), in which we have found the Bible texts, triggers these ambiguities.
We may see a classical example of the consequences of the scriptio continua in Gen 49:10 (see Emanuel Tov, Textual Criticism of thr Hebrew Bible, p. 3, ft. 2).
As regards the question presented by Collen (Num 26:10), we may say that a number of translations operate a link between the first part of the verse (“And the earth opened her mouth, and swallowed them up”, KJV), and the second part of it (“Korah, when that company died, what time the fire devoured two hundred and fifty men: and they became a sign.”, ibid.), compounding the verse in the following way: “And the earth opened her mouth, and swallowed them up together with [ואת] Korah, when that company died, what time the fire devoured two hundred and fifty men: and they became a sign.” (ibid.).
From a linguistic viewpoint this translation is correct.
Nevertheless, the collation of all the passages speaking about the end of those rebel ones (along with their supporters) may permit us to arrive to an another reading, that is, Korah died through a fire from God, whereas the other two died through a God-operated splitting of the soil.
Without be dogmatic, I allow some commentators explain themselves this possibility.
Joseph Benson, Commentary on the Old and New Testament (my emphasis):
“Together with Korah — These words seem to import that Korah was swallowed up with Dathan and Abiram. But it being more probable, as was observed on Num 16:32, that Korah was consumed by the fire of God with those who offered incense, the words may be otherwise translated thus: ‘The earth swallowed them up; namely, Dathan and Abiram; and as for Korah, he died with that company, what time the fire devoured, &c.’ — To the same purpose speak the Samaritan text and Josephus. “Also we have the testimony of the apostolical constitutions, and in effect of Clement of Rome, of Ignatius and Eusebius, that the Septuagint version originally gave the same account.” — Whiston. Junius and Tremellius render the passage, The earth swallowed them up, and what things belonged to Korah; namely, his tent, and goods, and family, his children excepted, as they are here. The psalmist seems to confirm the opinion that Korah was burned with his Levites, mentioning only Dathan and Abiram as being swallowed up, Psa 106:17.”
Adam Clarke, Commentary on the Bible (my emphasis):
“Together with Korah - The Samaritan text does not intimate that Korah was swallowed up, but that he was burnt, as appears in fact to have been the case. And the earth swallowed them up, what time that company died; and the fire devoured Korah with the two hundred and fifty men, who became a sign.”
To separate the two parts of the verse is not an odd way to translate. In fact, other passages presents a similar introductive structure (as regards what is said previously), that triggers a separation between what is said before ואת as regards what is said after.
A good example of this structure is Numbers 3:46, that many translation agree to convey: “And for…”, “But for…”, “And as…”, making a clear-cut separation as regards what is said before.
Similar reading ways we may found in Num 35:6, Eze 17:21.
So, a more consistent (from my viewpoint) translation should be:
“The earth open his mouth and swallowed them up [Datan and Abiram].
And as for [ואת] Korah, he died with his supporters when the fire consummed…”