how can we access variables of one workflow from another workflow

Not sure your exact requirement, but if it is related to change Management Workflow, then you can use changehelper2 to find related Object. Once you have object then you get related process using WfEngineHelper.service.getAssociatedProcesses API , once you have process the use below API's to get the process variable and their values

  WfProcess process = (WfProcess) pro;

  WfVariable vars[] = process.getContext().getVariableList();

  HashMap<String, Object> varMap = new HashMap<String, Object>();

  for (WfVariable var : vars) {

  System.out.println("Printing Variable " + var.getName() + "---value---" + var.getValue());

  }

Hope it helps!!!

Thanks

Shreyas