ARM technical documents don't mention shadow specifically, so it is kinda hard to relate it to ARM.
However from a comparision of ARM to MIPS architecture point of view:
The MIPS architecture supports the implementation of multiple “shadow” banks of registers. This allows more efficient context-switching operations but the fact that this feature is not commonly or consistently implemented on MIPS-based devices makes it of limited use. Consequently, few compilers or operating systems make use of it.
In this context "shadow registers" are more kinda stand free thing - they can be utilized for many things, while banked registers are for clearly separated execution modes.