Since BirdieSync needs to be installed on both the desktop and the mobile device to works properly, it is necessary to install it on your second mobile device. There are two possibilities.
You can fully reinstall BirdieSync while connecting your additional mobile device. That should install BirdieSync on the additional mobile device.
You can also manually install the BirdieSync mobile part on your additional mobile device. To do it, go in BirdieSync "Installation" desktop sub-directory (usually C:\Program Files\BirdieSync\Installation). Choose the appropriate cab file according to your mobile device operating system (for instance Setup.PocketPC5.0.CAB for a Pocket PC under Windows Mobile 5.0 or 6.0)). Note also that a Smartphone has no touch screen and that a Pocket PC has a touch screen. Then copy and paste it on your mobile device in \Program Files for instance. Then execute this cab file to install BirdieSync mobile part. Then reconnect your mobile device.