Solubility in homogenous mixtures of solvents Alcohol and water are miscible.  I am coming across some chemicals that are soluble in alcohol and insoluble in water.  If I know solubility in two solvents separately, is solubility in a mixture of the two always straightforward, or are there confounding variables?

E.g., if some chemical will dissolve to 100g/L alcohol at 25&deg;C, then will its solubility in a 50/50 mixture by volume of water and alcohol at 25&deg;C always be 50g/L?

Does this generalize to all miscible solvents?
The complication comes from solubility of the solvents with each other whilst simultaneously having some solvation power of a solute. I have made a compound before that was so water soluble that it caused sodium chloride to precipitate out. A real example was of an organic compound that had 1%w/v solubility in ethanol but around 6.5%w/v in toluene. In a 50:50 mixture,  the compound had solubility of 13%w/v. A completely non-linear result! The solubility was even higher in 90:10 toluene-ethanol. So, the complex nature of three way interactions mean it would be very difficult to predict these solubility graphs just using two data points of the two pure solvents. 
If we asked the same question of boiling points, we would point straightaway to azeotropes. So, analogously, other properties exhibit this behaviour. However,  in this instance the nature of the solute as a third component makes it too complicated to predict without having the wealth of property data available for all components. If this is known and you have the software, you may have a chance.