Large-aperture high-field superconducting (SC) magnets are used in various accelerator systems of particle accelerators/colliders. Large Lorentz forces and mechanical stresses can degrade or damage the brittle SC coils. The stress-managed cos-theta (SMCT) coil is a new concept for high-field and/or large-aperture accelerator magnets based on low-temperature and high-temperature superconductors. This concept was proposed and is being developed at Fermilab in the framework of the US Magnet Development Program (US-MDP). The SMCT structure is used to reduce the large coil deformations under Lorentz forces and, thus, the excessively large strains and stresses in the coil. A 120-mm aperture two-layer Nb 3 Sn SMCT dipole coil has been developed at Fermilab to demonstrate the SM concept including coil design, fabrication technology, and performance. The first SMCT demo coil was assembled with a 60-mm aperture Nb 3 Sn coil inside a dipole mirror configuration to be tested separately and in series with the insert coil. This article summarizes the SMCT coil design and parameters, the mirror magnet assembly, and the results of separate SMCT coil test in the dipole mirror configuration.