expression, suru verb (irregular)
fold in two; fold in half; double up
Used for folding paper, cloth, or other flat items exactly in half. The expression is a set phrase combining 二つ折り (fold in two) with にする.
紙を二つ折りにして封筒に入れてください。
Please fold the paper in half and put it in the envelope.
このチラシは二つ折りにすると
If you fold this flyer in half, it will fit in your pocket.