Javascript operates in a single thread, so in any scenario they would be run sequentially.
What makes you require them to be executed at the same time? Why not join them into one ajax request by joining the two scripts together?
Another option is to delay the execution of the script you're calling by putting a sleep
in it or something. But I really would not recommend this. The best I would suggest is to make one script do what you need instead of both.
If it is a DB operation that requires data simultaneity, you could use transact style commands to ensure that there isn't a nanosecond where there are data discrepancies. This is better because for whatever reason you need them to be called simultaneously, one could alter javascript on the client end very easily from an end user point of view. This is why its better to do it on the server.