Try this

A _ 0.
B _ 0.
[ [A < 50] whileTrue:[
        Transcript show:'AAA';cr.
		Processor yield.
        B_ B+1.].
] fork.

[ [B < 50] whileTrue:[
        Transcript show:'BBB';cr.
		Processor yield.
        A _ A+1.].
] fork.

Basically you have to give other processes a chance to run. 
This is what Processor yield does. 

Markus

-- 
+----------------------------------------------------------------------------+
| Markus Kohler                          Hewlett-Packard
GmbH                |
| Software Engineer                      OpenView Software
Division          | 
|                                        IT/E Response
Team                  |
+----------------------------------------------------------------------------+