Some among the beleagured human and demihuman communities see this as an opportunity to make common cause with the yuan-ti against the lizardmen. However, it is clear that the yuan-ti do not see any human or demihuman person as anything close to an equal--they are at best cattle: valuable enough while alive, but disposable and useful only as a resource to be consumed.
Some among humanity know this, and stand as firmly against the serpents as they do against the lizards. Others know this, and think that they can exploit this fracture between the reptiles to their gain, keeping a wary eye on the yuan-ti. Others know this and think that being the cattle of the yuan-ti is better than being dead on the blades of the lizards. And finally, some actually trust the yuan-ti's promises of aid and succor. These poor dupes were once a fringe in human and demihuman society, but their numbers are growing, particularly as yuan-ti secretively infiltrate more and more of it. And this divisiveness among the "warm bloods" is a wedge that the yuan-ti can exploit to infiltrate even more deeply.
Like the lizardmen kingdoms, the yuan-ti are divided into several city-state or larger entities, which have a fierce rivalry with each other, but which are reluctant to show lack of unity to their enemies. Often, embassies, or even armies, are designated by multiple such nations to speak on behalf of the entirety of yuan-ti-dom. But it is in the snakes' nature to constantly attempt to deceive and manipulate each other as well as their enemies for their own gain. Many of their cities are in the drier parts of the continent, alongside rivers. The yuan-ti require little in the way of water to survive, although some live deep in constantly humid rainforests or swamps as well.
The lizardmen collectively control about half of the territory on the continent. The yuan-ti, about a third. That leaves (roughly) 15% that is either too remote and resourceless to be of use to anyone, or under the control of the warm-bloods. The yuan-ti, generally, would like to reverse some of those percentages, with them controlling half of the territory, and humanity taking over a larger chunk, although under their own watchful eyes. While temporarily useful, the yuan-ti have no long-term use for the lizardmen at all, and would not weep to see them completely eradicated. They know, of course, that such a goal is a long-term one, as the lizardmen today are the strongest group on the continent by far.