Now we have 2 TFS servers, one is for code and one is for work item tracking. Now we are trying to enhance our process to the code review using the TFS code review work flow in VS2013. Since the work item tfs and code tfs is not exactly the same one , is there any way to implement this instead of migrating one of the tfs servers?
No. If you want your work items and your source code to be related to each other, they need to be hosted in the same instance of TFS. Ideally, they'd be in the same team project. Think of a team project as a portfolio of related artifacts representing the complete SDLC from planning through to deploying.
The setup you have right now makes no sense. There is no conceivable benefit to having work items and source code in separate instances.