To Stitch Two Meshes Together Using the Merge Edge Method

Select the Stitch into Mesh tool ( Modeling > Mesh > Modify Meshes ).
Select Merge Edge.
(Optional) Select Boundary Segment.
Select the first mesh element.
Do one of the following:

Select the desired boundary.

or
If Boundary Segment is selected, select a start point and end point for the segment.

Selected boundary or boundary segment displays with a yellow highlight.
Select the second mesh element.
Do one of the following:

Select the desired boundary.

or
If Boundary Segment is selected, select a start point and end point for the segment.

Selected boundary or boundary segment displays with a yellow highlight.
Click to merge the edges of the two mesh elements together.

Merge Edge method with Boundary Segment selected