The Institute of High Energy Physics (IHEP, China) has been engaged in the development of shell-based dipole magnet with common-coil configuration for the pre-study of Super proton-proton Collider (SppC) project. The first subscale magnet LPF1, with two Nb3Sn coils and four NbTi coils, reached a bore field of 10.2 T at 4.2 K. Then a higher safety margin model has been proposed as LPF2, which has six Nb3Sn coils and two NbTi coils to reach a 12-T main field in both apertures with an operating current of 5300 A. The shell and yoke are reused from LPF1, as well as the two Nb3Sn coils and two inner NbTi coils of LPF1. The pre-loaded method of LPF2 is much alike that in LPF1: using Bladder & Key technology to overcome the Lorenz force in horizontal and vertical direction and pre-tightening six aluminum rod to pre-load the coil packs in the axial direction. While, the strain gauges are applied both at the surface of the aluminum shell and the inner surface of the innermost coils to monitor the pre-loading effect. The main design parameters, stress analysis, assembly procedure and strain measurement results during the assembly and test at 4.2 K will be presented.