Basically, after:

hg clone h.p.o/cpython default
hg share default 3.4
hg share default 2.7
hg -R 3.4 update 3.4
hg -R 2.7 update 2.7

the 2.7, 3.4, and default directories are separate working copies created from the same history, each at a different revision.  As soon as a changeset is created in one, it's available in the others (but the other working copies don't change).  So after commiting to 2.7 (from within the 2.7 dir), `cd ..\3.4 && hg graft 2.7` will try to graft your new changeset to 3.4, then `cd ..\default && hg merge 3.4` will merge it with default.  hg push will push the same thing from any of the three directories.