Q:

How to weld two objects along a common seam?

Using the knife project tool I have been able to scale my entire mesh to TWO resolutions,  cut out (and delete) the ear from the low-resolution model, cut out the ear from the HIGH-resolution model (using the same vertex path), and graft it onto the low-resolution model:

The result is good; it is an almost perfect join.  Only in a few places can I see a tiny gap.
However, it is still 2 objects, and simply joining them doesn't connect the meshes.  I would like to connect the meshes so that I can smooth around the join line, and use the 'simplify' brush to transition gradually from big triangles to small triangles.
Is there any such tool?

A:

The Bridge tool is probably your best bet. The connection is going to have issues if there are a different number of vertices on each edge. Either, the number has to be the same, or there will be lots of triangles. These issues might not actually be problems, for what you want to do, but it's something to be aware of.

A:

Use LoopTools.
First move the ear away from the head, just to make it easier. Then join the two meshes with Ctrl-J. Then tab into Edit mode, select the two edges that will be connected. Use LoopTool's Bridge to join them. 
Then move the ear back where it belongs.

