Create the assembly directly from the Save Bodies PropertyManager.
Under Create Assembly, click Browse, select a folder, and save the assembly as AssemblyMultibody.sldasm.
The Save
as type box indicates this is a SplitAssembly
file type.
The full path to the assembly appears in the PropertyManager.
Click
.

Open AssemblyMultibody.sldasm.
The components use the same names as the bodies you saved from the multibody part.