The usual way in C to do this is to use a union.
If _AL and _BL both are int's I wonder why you need two separate variables? If the reason for the second variable is another type,
then using a cast may do the trick
#define _BL (*(signed int)&_AL)
As third thing to look at is the PAGED qualifier in the prm file.
different not initialized variables can be placed into different PAGED segments, that's actually the only purpose of the PAGED qualifier.
Daniel