With the Windows version of BIAB, users can right-click on the chordsheet (not a bar number) and then select "Bar Settings".
When in 'Bar Settings', change the number of beats to 2.
Also, you will need to set the following bar back to 4 beats by again using 'Bar Settings'. If you don't do this, the song will remain with 2 beats per bar until the end.
If I right click on a chord (say next to bar 19) and change "beats this bar" to 2 followed by right clicking the next chord (next to bar 20) and change "beats this bar" to 4 what happens is 19 plays half the bar (as I want) but then 20 starts with drums only for half a bar and the rest kicks in the second half of that bar