Can the state combine different cases against one defendant in one indictment?

Cases are eligible for consolidation if (1) the cases involve substantially similar factual or legal issues, or (2) parties are the same. Your attorney may decide whether to file a Motion to Sever, which may be granted when the court decides that the cases involved contain sufficiently different issues of fact and/or law that continued consolidation will not promote efficient case resolution.

If there is a legal basis, they can do it. There usually has to be a nexus (connection) between the two cases/charges..........

There are circumstances under which they would be able to consolidate the cases, but there would have to be some sort of connection between the two cases.

The concept you are referencing is called "joinder". The are several factors that will allow the prosecutor to join two cases together. For cases to be joined there must be a nexus between the charges.