Thanks for your answer.
Here's my friends statement in full:
The eBay advert [for Soubitez dynamos] doesn't give much information. If the unit really is a dynamo then it should be delivering Direct Current, for which some form of rectification would be built in, using either a solid state diode bridge or low tech armature and brushes. The output statement: "3w" appears to confirm this (technically, an alternator would be rated 3VA) but probably shouldn't be relied on.
I am guessing that the unit uses permanent magnets for a field, rather than using a self- excited field coil. The latter can be re-polarised to enable reverse rotation by "flashing" the field terminals with a battery.
Regarding the permanent magnet type: assuming that the mechanical drive within the unit can accept reverse rotation, all that happens when (e.g.) a L/H device is rotated R/H (whatever that means) is that the polarity of the output terminals is reversed. This needs to be noted and taken into account before two units are wired together in parallel.
However, if the unit is really just a simple alternator (bearing in mind that incandescent light bulbs can run on either AC or DC with no issues regarding polarity) then wiring a pair of them in parallel is likely to introduce insoluble phasing problems.
What I actually want to do is run the dynamos simultaneously into a battery to charge it, does that change the situation?