I have never had a problem of different python versions coexisting on
the same machine, but having 32-bit and 64-bit python coexist is much
harder. Particularly when 32-bit python is compiled on the 64-bit OS
(using -m32 flag). There is a related issue1294959 highlighting this
problem.
See also issue1536339, issue1553166, and issue858809.
Ideally, I would like to see a mechanism that would allow both standard
library and user modules to share machine independent (*.py{,c,o}) files
while having separate locations for 32-bit and 64-bit C modules.