Tested on Linux, and repeated the issue - just to confirm this isn't a
Solaris specific issue.

Tested using dummy_threading - bug does not appear when that is used.

Added _verbose=True to threads, and log_to_stderr(SUBDEBUG) set for Process.

Attached is the output of the script with those set. 

Other odd things to notice, and some point, more than 2 children
sometimes exist.. 

isalive PID: 7646 : True
isalive PID: 7652 : True
isalive PID: 7653 : True
isalive PID: 7646 : True
isalive PID: 7652 : True
isalive PID: 7653 : True
isalive PID: 7646 : True
isalive PID: 7653 : True

The code starts 2 threads, which start 2 processes. Both threads are
joined before new ones are started.. So, in theroy, only 2 should be
able to exist at once.. ?