Do you select your first mesh, click on a peripheral segment, choose the "+" option and finally press the space bar before clicking on the other mesh ?
This method will not merge meshes but will nevertheless make it possible to recover the influence of the second mesh. Otherwise I can advise you to hide the outline of the two meshes and then to redraw in 2D the global outline of the two meshes to create the illusion of a single mesh.
Or you can create a Patch to draw over this line. I agree with you it is not a great solution but you are able to get your aim by this way.