John Gill Commentary 1 Kings 16:21

"Then were the people of Israel divided into two parts: half of the people followed Tibni the son of Ginath, to make him king; and half followed Omri." — 1 Kings 16:21 (ASV)

Then were the people of Israel divided into two parts
About the succession in the kingdom:

half the people followed Tibni the son of Ginath, to make him king :
these were the friends of Zimri, or however such who did not like that the army should impose a king upon them; who this Tibni was is not said:

and half followed Omri ;
the general of the army, perhaps the whole of that.