multipath - parallel or multiplexed I/O?

Hi,
I'm a little bit confused.

I would like to ask if round-robin mode of dm-multipath on rhel6 imply that on any given time only one path is used (inside its path group) for the occurring I/O.

As far as I know multipath cannot do "parallel" or "simultaneous" I/O over multiple paths, instead it can read or write from only one path at time, but I may be wrong because other colleagues pointed out that one of the multipath purpouse is to spread I/O over available paths, so it sholud be done in "parallel".

My guess is that is true, meaning that all available paths are used, but in a "multiplexed" fashion rather than "at the same time".
I'm not able to explicitly get confirmation over this on the official documentation.

Thanks in advance.