How can co-defendants have a case seperated?

[helpmeplease]

Is there a way that co-defendants can have their case seperated? This would be a felony case where the D.A. is trying to keep the 2 as a packaged deal only because one of the defendants is the father to the 2nd defendant and has some previous run ins with the law. The other defendant has no criminal background and has never even been arrested so, they are trying to use this as leverage over the father to get him to go ahead and plea to felony grand theft, or felony possesing stolen property. If the case goes to trial there is a very good possibility that these charges can be beat. The problem is that if it goes to trial then the defendant without a criminal record would not be eligible for pc1000 diversion and only would I guess be able to do prop. 36 for the possesion of controlled substance charge that they are also being charged with. What would be the best thing to do in this situation for both of the defendants? They both were not guilty of the theft or actually the possesing stolen property however the defendant without the background was in possesion of the controlled substance. It doesnt actually seem very fair or even legal that this be allowed to be that way. What options do these people have if any at all?