I can see why you may want to route the isolated spotify through Winamp, but Media Player can't do anything that Winamp cannot.
That makes no sense at all, unless you are having problems with DRM content maybe.
If you have a desktop then you may find that your multi-channel sound card can split the front and rear as 2 separate outputs.
This is a common feature in modern Realtek drivers.
Using a replacement Volume control panel can help allocate the correct ins and outs
Volume² -
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