This isn't really a
servlet question, since the "posting" part is on the client. There's no special code required for servlets to handle multiple concurrent requests because it's build into the architecture.
So it would have been better to ask in one of the more advanced General
Java forums.
Off the top of my head, posting parallel is not a problem, but I'd have to RTFM to see how to keep from hanging once you were ready to receive the responses. In the normal course of events, responses would all come back to a single port and you could just listen for incoming response traffic on it, but HTTP assigns a different, randomly-selected reply port to each outgoing request.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.