This is going to be tough to achieve with two sculpted objects, its not impossible, but will require an extensive amount of work.
Ideally you want the same number of verts on the the two edge loops on the face and on the mouth.
I would suggest the best option would be, join into one mesh with CTRL+J, and retopologise over the base mesh.
Or
Retopology first on both meshes, then CTRL+J to join them to one mesh, and then merge each vert around the edge loops.
