I'm able to reliably reproduce this bug (using Apache 2.2.8, otherwise
same as above), although not with mod_python's simple tests, but only
with Trac (apparently, Trac creates some threads while processing the
request).

How to reproduce: configure two Trac/mod_python locations in Apache
config and set them to use different Python interpreters:

    <Location /trac1>
       SetHandler mod_python
       PythonHandler trac.web.modpython_frontend 
       PythonOption TracEnv /srv/bakefile/trac
       PythonOption TracUriRoot /
       PythonInterpreter trac1
    </Location>
    <Location /trac2>
       SetHandler mod_python
       PythonHandler trac.web.modpython_frontend 
       PythonOption TracEnv /srv/bakefile/trac
       PythonOption TracUriRoot /
       PythonInterpreter trac2
    </Location>

(As far as this bug is concerted, this is the same as having two virtual
hosts that both run Trac -- mod_python's default interpreter has the
same name as the (virtual) host.)

Then run Apache as "apache2 -X" to ensure that requests are handled by
single handler serially and do

  $ curl http://your-server/trac1/wiki
  $ curl http://your-server/trac2/wiki

The second command crashes Apache.

If you change mod_python configuration to use the same interpreter names
for both Trac instances, the crash doesn't happen (but of course, that
prevents you from using different versions of Python modules in both
vhosts).

I'm attaching Valgrind log, but it's not very useful -- it's not deep
enough and my server doesn't have enough memory for high enough value of
--num-callers.