When Oliver Cowdery copied from 𝓞 into 𝓟, he initially omitted “and Lamoni”, perhaps because the associated verb form was the singular was (“as Ammon … was a journeying thither”). Oliver soon caught his error and supralinearly inserted & Lamoni (the level of ink flow for the correction is slightly heavier, perhaps because he redipped his quill before making the correction). It is theoretically possible that the following plural pronoun they led Oliver to emend the preceding text by inserting “and Lamoni”, but the critical text will assume that the corrected reading in 𝓟 represents the original reading. Of course, the use of the singular form was with plural subjects is quite prevalent in the original text, as discussed under 1 Nephi 4:4.
Summary: Maintain in Alma 20:8 the corrected reading in 𝓟 (namely, the conjunctive subject in “as Ammon and Lamoni was a journeying thither”).