Typically you should sculpt all meshes you wish to tween from the same start or make them by deforming a base. Blending meshes comes from the film and game industry (in Maya it is called blend shape) where typical practice is something like sculpting a base face and then moving verts around to make different face expressions (while not adding new verts or faces). Then you can blend through the face expressions. It is not really enough to just have the same specific amount of verts and faces but to have the same topology, otherwise you won't usually get things tweening to the places you want them to. 

As it is explained in Autodesk Maya's blend shape tip "If the target and base objects have the same number of vertices but their order is different, Maya blends the shapes whether Check Topology is on or off. However, the position of the base object's vertices will be transformed to the position of the target objects' vertices. This change might cause the object to blend in a way you might not expect. To ensure a smooth transition between base and target, make sure the order of vertices is the same in both base and target objects or you might see some unusual blending, such as an eye blending into the nose."

If you just want to get the same counts and don't care what blends to what I usually use ZBrush to retopologize.