Create your topic branch off of the first branch. As soon as the first is merged into master you can rebase on top of that, and assuming not too much was changed it shouldn’t be a problem.
If the commits of the first branch aren’t changed your new branch will stack neatly on top of that, and if the commits are changed (squashed, edited or whatever) you can always do an interactive rebase of the second branch and edit it to look good once the first branch has been merged.