If it is about pushing changes to Git, then I think there are always enough other committers around who can push those changes, even if it is not 'their' package. And I did this many times for others when they were asking for a change, a removal, or an addition on a bug report.
If it is about testing and voting (I guess that's the main point of your question), then you just need to look into the mail archives that I always took +1 votes from trusted persons other than the 'maintainer'. Examples are the Testing and the Scout packages; for both packages we receive votes on the mailing list from persons that I trust to have the knowledge and the authority to give a +1, but they are not committers on the project.
For me it is important to have a single person who is responsible for a package and that this person can push changes to Git, but that doesn't mean that there cannot be additional package testers. In fact some packages are listing many on the packages page (e.g. [1] on the bottom right). My fear is that it wouldn't solve any problem if there was more than one committer per package, it just adds more overhead. In my opinion the main problem is the (limited) testing itself that could be improved.
[1]
http://www.eclipse.org/downloads/packages/node/1089
Thanks,
Markus