The ONLY reason I can see for simultaneous tests would be to so contaminate the outgassing resulting from the test (some does happen even with an underground test) that it would be difficult to determine the source of the material used in making the explosive device (ie: was the uranium from Iranian mines etc.).

IF there are two simultaneous, or near simultaneous tests then it is very plausible that they are doing a test on behalf of Iran, OR some other country.