Is there a way to lock branches in GIT?

Hi GIT Gurus,

1. If there are 2 user are working and having their own clone and branches
(not master, other branches). and there is another parent clone with a
branch (not master). Can these 2 users merge to the parent clone branch at
the same time? or only one user will be able to merge first and then the
other. in other words, will the target branch be checkedout to that user
alone and other user cant checkout that branch or use the checked out
branch. 

2. Is there a way to lock branches, so that if a user want to merge his
changes to a parent branch of another clone, it can be unlocked for him
alone? Hope you can help me on this. 

Thanks ask4thunder