Is it possible to view multiple git branches at the same time for the same project?

You can simply copy the repository to a new location (either by literally copying the directory, or using git clone --shared) and check out one branch per location.

You can also use git-worktree for creating multiple working directories from a single instance of a repository.

Otherwise, the primary means for comparing files between branches prior to merging them is git diff.

With Git 2.5+ (Q2 2015), a Git repo will support multiple working trees with git worktree add <path> (and that will replace contrib/workdir/git-new-workdir)

Those "linked" working trees are actually recorded in the main repo new $GIT_DIR/worktrees folder (so that work on any OS, including Windows).

See more at "Multiple working directories with Git?"

Yes it is possible with appropriate care. However you are taking one of the copies 'away' from the regular git directory using --work-tree=<path> option, so changes there won't be seen by git unless you specially tell it. I gave an example here single-working-branch-with-git - see the UPDATED segment.

Note that the git-new-workdir doesn't work on Windows XP as it requires Unix style links.

First thing that comes to my mind it to checkout each branch on separate project. So: 1. checkout branch A on primary clone (1) 2. create a new clone (2) 3. checkout branch B in clone 2

Second approach could be to create a new branch (aka C) and merge both branch A and B to it. If they are complimentary than this might help with your review.

As already mentioned, you can diff branches with git diff:

git diff [--options] <commit> [--] [<path>…]

This form is to view the changes you have in your working tree relative to the named <commit>. You can use HEAD to compare it with the latest commit, or a branch name to compare with the tip of a different branch.

Excerpt above is from Git documentation.

Now git includes the command worktree to do exactly that.