Is it possible to use 2 Bluetooth Unidirectional devices simultaneously? Is it possible to pair (and use) both a microphone-only and a speaker-only Bluetooth devices so that they can be used instead of the built-in mic/speaker in the phone?

If so, can you point to where I can find more information about how to accomplish that?
It should be possible to do what you want. Try this: pair both of the devices with your phone, then go to Settings -> Wireless &amp; networks -> Bluetooth Settings, and long-press on the paired device and  select "Options".  From there you can choose which service you'd like for this particular device.