It seems that a large number of Jews in the time of the book of M'lachim served both the Baal and God. Most famously, perhaps, is Ⅰ M'lachim 18:21:
עַד מָתַי אַתֶּם פֹּסְחִים עַל שְׁתֵּי הַסְּעִפִּים אִם ה׳ הָאֱלֹהִים לְכוּ אַחֲרָיו וְאִם הַבַּעַל לְכוּ אַחֲרָיו
How long halt ye between two opinions? if the LORD be God, follow Him; but if Baal, follow him.
Rashi and Radak there explain that the people couldn't decide which god to serve. M'tzudas David say they served sometimes one and sometimes the other; presumably Rashi and Radak agree to this.
Some half a century later, we have Yehu trying to kill out all the Baal-worshipers. Ⅱ M'lachim 10:23:
חַפְּשׂוּ וּרְאוּ פֶּן יֶשׁ פֹּה עִמָּכֶם מֵעַבְדֵי ה׳ כִּי אִם עֹבְדֵי הַבַּעַל לְבַדָּם
Search, and look that there be here with you none of the servants of the LORD, but the worshippers of Baal only.
That verse implies that, now, the two groups — that of God-worshipers and that of (l'havdil) Baal-worshipers — no longer overlap. Why not? What has happened in the interim?
(And claiming that the events of Ⅰ M'lachim 18 were successful in eliminating those who worshiped both God and Baal will require good evidence or a good argument. We see it didn't eliminate Baal-worshipers, so I see no reason to think it eliminated fence-straddlers.)
 Note that "only" in the translation applies to "worshippers of Baal" (the only people present should be worshipers of Baal) and not to "Baal" (the people present should be worshipers of only Baal). This is clear in the Hebrew, which uses a plural word for "only".