How to synchronize multiple xPC target machines?

I am to use two xPC target machines and encountered the problem unsolved for a long time: how to synchronize them?

After searching on Internet, I found to achieve this the following factor should be carefully considered: (1)start different targets at the same time; (2) keep their sample rates aligned; (3) for a longer run application, master-slave mode for them is a good choice ( http://www.mathworks.cn/support/solutions/en/data/1-1BR95/index.html?solution=1-1BR95).

However, I am not clear how to achieve (1) and (2). As some hardware for (3) is necessary, I would like to try (1) and (2) firstly.

Could you please give me some tips or some reference? Thanks a million.