When you split an assignment into segments you can set the amount of work and the schedule dates for each segment individually. 

You split an assignment into segments by adding new segments to a row or by splitting an existing segment.

Previously split work can be merged.

Split an existing segment

Select the work you want to split by clicking the bar. One week segments and segments tied to sprint dates cannot be split.

Then select the split icon.

Select at what week you want to split the work. Two week segments will always be split in half.