I "don't say" ... :-D ... ok, it seems as I wrote two pages for you exclusively, without saying anything.
Please understand that it's not my job to explain users the handling of their DAWs. So let's start from the opposite end, once again: How would _you_ set up a surround session in Reaper?
If you would do so, you could use _any_ downmixing device (i.e.: multi-channel to stereo) of your choice for the dynamic mixture of up to eight individual MIR Pro output channels. Is this really so hard to understand? ... must be a language thing.
/Dietz - Vienna Symphonic Library