You could make the commit on one branch, switch to the other branch and cherry-pick that same commit to apply it to the other. But, you'll likely get a conflict if you try to merge the branches later since they both make a change to the same lines.
If it's possible to make a third branch that starts before the second and make the change there, then that thirds branch could be merged to both the other branches and won't conflict later when they're merged.
-- Tony