ChanSpy/Monitor multiple channels via one extension

Hello,

This is my starting point:

extension x (radioDevice_1) <- is connected with -> meetme conference 123
extension y (radioDevice_2) <- is connected with -> meetme conference 456

ChanSpy a <- is monitoring -> extension x (radioDevice_1)
ChanSpy b <- is monitoring -> extension y (radioDevice_2)

But what I want to accomplish is:
extension z <- is monitoring both radio devices (over ChanSpys e.g.) -> extension x + y (concurrently - mixed audio signal)

I know in theorie that I can accomplish it by conferencing multiple SIP calls on client side.
But a much better way would be a server side solution in asterisk, I think.

Thank you very much for your sugestions and hints.
Best regards

PS. I found another thread regarding this issue in the forum (sorry for that ): search for “Originate from local channel failing”