ConstrainAB

"This script Position and Orientation constrains one object between two others - with an animateable Custom Attribute for mixing between the two.
This script works after you select 3 objects. The 3rd selected 'slave' object is pos and rot constrained to the first two 'masterA' and 'masterB' selected objects. The slave object gets a Custom Attribute that allows you to mix between the two constraints. "