Bass clarinet and tenor sax share the same soundfont?

First it helps to get the terminology right - a soundfont is a set of sounds for all instruments (well, it can be for all, anyhow, and is by default). So, yes, bass clarinet and tenor sax use the same soundfont, so does flute, xylophone, and every other instrument. If you mena they use the same sound within the soundfont, that shouldn't be - bass clarinet should be using the clarinet sound. That's because the General MIDI standard, used by most soundfonts, doesn't include it as separate instrument. So you'll have to find a specialty (non-GM) sounfont that does include this instrument and then load that and use it as described in the Handbook.