It is now possible to either split a moment in two, or Merge a moment into another.
If you would like to split a moment click on the drop-down menu in the right-hand corner (icon with 3 dots). Choose "Split a moment in two" and a scissor icon will appear next to the photos. Then select the photo where you would like to split the moment.
You can also merge a moment into another moment. To do this, click on the drop-down menu in the right-hand corner and select "Merge with other moment"