You can add multiple bluetooth  dongles to usb ports of your computer. But, in my experience:

- windows sucks at managing multiple bluetooth devices, it just doesn't work sometimes.
- linux has a great bluetooth stack with excellent support for multiple bluetooth devices: bluez. I highly recommend it for any serious work. Just try some of the bluez commands and dig deeper :)
- I don't recommend the bluetooth JSR. The implementations are not so good and it's limited. Just fire bluez commands.
- multiple devices doing parallel sends/scans just doesn't work. It causes noise and slows down all devices. Bluetooth really is not a very scalable platform for very parallel operations. Have fun making it work