I believe only one program can access an ASIO driver at a time. Sonar probably has some setting on the page where you select the drivers to do something like "release ASIO drivers in background." This is the language Cubase uses. Then when Cubase is not longer the active window, i.e., you have BIAB or another application on top and 'open,' the drivers will be released to the 'on top' application. Most DAWs have some setting that performs this function.

However, I have no idea whether or not BIAB also has a setting for the capacity to release the driver back to Sonar, when Sonar has the active window.

Prado