The name of this city seems to be a compound composed of Jacob and Ugath. (There is the similar proper noun Ogath in the Book of Mormon, found in Ether 15:10.) The question here is whether this city name should be spelled as a compound or as a single word. The original spelling in the printer’s manuscript (Jacob Ugath) suggests a compound spelling, while the 1830 compositor’s spelling of this city as Jacobugath argues for the single-word spelling. Later in 𝓟, Oliver Cowdery inserted a hyphen between Jacob and Ugath, giving Jacob-Ugath; the level of ink flow for the hyphen is somewhat heavier. It is also possible that this ink stroke was an attempt to connect Jacob and Ugath into a single word (see the note for line 10 on page 378 of 𝓟 in volume 2 of the critical text).
There are several instances of compound names elsewhere in the Book of Mormon text. Consider first of all the spellings in 𝓟 and in the 1830 edition for the two names that derive from sentences in the original Hebrew, Shear-jashub and Maher-shalal-hash-baz (Isaiah’s names for his sons):
𝓞 is not extant for either of these two names, but their original spellings in 𝓟 suggest that in 𝓞 there were spaces between the morphemes but no hyphens. For both names, Oliver Cowdery later inserted the hyphens in 𝓟. Note, however, that for the first name the 1830 compositor rejected Oliver’s two-word spelling Shear-Jashub and set the name as a single word, Shearjashub. This example shows that here in 3 Nephi 9:9 the compositor’s one-word spelling Jacobugath could derive from a two-word spelling in 𝓞, either Jacob Ugath or Jacob ugath. (It is rather unlikely that the reading in 𝓞 had a hyphen separating the two morphemes since Oliver initially copied the name into 𝓟 with only a space between the two morphemes.) Of course, the 1830 spelling Jacobugath could derive directly from an original single-word spelling in 𝓞 (in other words, Jacobugath).
The Book of Mormon also has two compound names involving Lehi and Nephi. The first is the name Lehi-Nephi, a land and its capital city. Out of seven occurrences of this name, only the first one (in Mosiah 7:1) is written in 𝓟 with a hyphen (as Lehi-Nephi); the remaining six (from Mosiah 7:1 through Mosiah 9:8) are written LehiNephi in 𝓟. As expected, the 1830 compositor supplied the hyphens for these six other cases. 𝓞 is not extant for the book of Mosiah, but 𝓞 probably read LehiNephi in all seven cases. This conclusion is supported by the consistent manuscript spelling for the second compound name, Anti-Nephi-Lehi. This name occurs 12 times in the text, and in each instance it is spelled without any spaces or hyphens in 𝓞 (where extant) and in 𝓟—that is, as AntiNephiLehi (from Alma 23:17 through Alma 43:11). In all 12 cases, the 1830 compositor set the name with hyphens, as Anti-Nephi-Lehi.
Probably the most relevant compound name for analyzing the city Jacob-Ugath is the name for the village Ani-Anti:
As explained under Alma 21:11, the most likely spelling for Ani-Anti in 𝓞 was the two-word spelling Ani anti, which Oliver Cowdery initially spelled in 𝓟 as one word, Anianti. Oliver immediately corrected this one-word spelling to the two-word Ani Anti by crossing out Anianti and writing inline Ani Anti. The space between the two morphemes and the capitalized A for Anti indicate that Oliver intended this name to be a compound. Later, with somewhat heavier ink flow, Oliver inserted a hyphen between the Ani and Anti. These corrections in Alma 21:11 argue that here in 3 Nephi 9:9 the name Jacob-Ugath probably read Jacob ugath in 𝓞, which Oliver initially copied as Jacob Ugath into 𝓟 but the 1830 compositor set as Jacobugath.
Since there seems to be a connection between the name Jacobugath and its founding king Jacob (mentioned here in this verse, also earlier in 3 Nephi 7:9–13), I would recommend showing this relationship more clearly by placing a hyphen between Jacob and ugath. For compound names where there appears to be an equal conjoining of morphemes, the critical text will capitalize the noninitial morpheme(s) as well as the initial one, thus the names Lehi-Nephi and Anti-NephiLehi. Following this pattern, the critical text will capitalize the noninitial morpheme Ugath in the city name Jacob-Ugath, similarly for the noninitial morpheme Anti in the village name Ani-Anti. The names for Isaiah’s two sons derive from sentences in Hebrew and are therefore spelled with hyphens, but for those cases the noninitial morphemes of the compound name are not capitalized (see under 2 Nephi 17:3 for the name Shear-jashub and under 2 Nephi 18:1 for the name Mahershalal-hash-baz).
Summary: Change in 3 Nephi 9:9 the 1830 compositor’s spelling Jacobugath to Jacob-Ugath, in accord with the initial spelling Jacob Ugath and the corrected spelling Jacob-Ugath in the printer’s manuscript; this spelling parallels the spelling of the compound name for the village Ani-Anti mentioned in Alma 21:11.