They've give you the option to put your own version of Gmail back into beta, you know, if you're into that sort of thing:
Why would you want the withdrawal and balance check to run concurrently?
Because it might be too much trouble to get both processes to run on the same node. With proper barriers, you can make sure they get run in the correct order.
"The eleventh commandment was `Thou Shalt Compute' or `Thou Shalt Not Compute' -- I forget which." -- Epigrams in Programming, ACM SIGPLAN Sept. 1982