Crystallization issue
Under the right conditions, crystallization can be a very effective means of obtaining a pure solid form of the solute. Unfortunately, in your case one of those conditions is violated; you have a second solute with similar solubility characteristics.  You didn't quantify the solubilities of the solutes, but the phrase "very close solubility in a certain solvent" tells me that under the conditions of your system, crystallization will not be an effective means of obtaining a pure solid form of the solute.  

One option, as mentioned in a comment to your question, is to choose a different solvent. Selecting the right solvent / solvent mix can be a bit of an art and will also require some research and experimentation on your part.  Because you didn't discuss the specific solvents or solutes used in your system, it is not really possible to give more specific advice on solvent selection.   

Another option is to choose a different means of solute separation, like some type of chromatography. Again, not knowing anything about your compounds makes it difficult/impossible to give further advice regarding this technique.