Quoting Commentary for I Chronicles 16:27

ויחד, a word derived from the category חדה, similar to the word חדוה in Chronicles I 16,27 עוז וחדוה, “strength and joy.” The construction follows the rule applicable to all roots of the ב,ג,ד,כ,פ,ת group of consonants (letters which sometimes take a dagesh chazak). They also lose their last root letter as a result of this. (in our verse the letter ה) More examples are the root בכה and the construction ויבך, vayevk, “he cried.” Another well known example is the root שבה, and the construction וישב, vayashav, “he returned.”

ויחד יתרו, “Yitro rejoiced over the goodness, etc.” The word ויחד is the same as the word חדוה in עז וחדוה במקומו, “strength and joy are in His place” (Chronicles I 16,27). We may also see the word אחדות as intimately related to the word ויחד, “he united.” This meaning would reflect that Yitro had converted and accepted the אחדות, the “Unity” of G’d. He blessed the name of the Lord as we hear in verse 10: “blessed be the Lord who has saved you .”