Patch Set 1:

This can be done using git rebase -i, see e.g. here :
http://www.gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html
ensure that the squashed commit contains the ChangeId of the first change before you upload it to Gerrit.

If you want to fix a patch already pushed to Gerrit use commit --amend to replace the original commit and push it again. But if you already committed this second change git rebase -i should do the trick.

Another handy tool is gerrit-cherry-pick, e.g. see here :
http://dev.eclipse.org/mhonarc/lists/egit-dev/msg00473.html