Shortened last measure in a score

First delete the section break (what you call "new score" break). The sections will join up but don't worry, we will separate them again later. Now click on the first measure of your second section and from the insert menu insert a horizontal frame. Then add the section break to that frame. This will put things back where they were but with a gap at the end of that first section. You can control the length of that gap with the handle on the right of the frame. Somewhat counterintuitively if you try to drag the handle to the right you will find it is the left margin of the frame that moves to the left. (What is happening is that you are lengthening the frame but as it can't extend beyond the page margin it is the left frame margin that moves.)