What is the solvent? If they are both in water, or if they are both in oil, then you can mix them. If you are asking about toxic gas emissions, no, you won't get any of those from mixing paint that you wouldn't get from mixing two containers of the same paint. However, the two paints may not be compatible with each other, and you may end up having the pigments crash out of the paint or getting a paint that won't dry or that won't stick to the surface.