To answer your question yes.
You can go half and half on risers, but it’s best advised to just go all the way as it makes cooling the cards if you planned in doing so much easier than having to do double the work.
To help you out long term, not sure if you’re aware but that’s a way overvalued mobo.
There are a load of others half the price of the m7, with 2 being just over a third less with an additional 7th lane:
https://forum.zcashcommunity.com/t/lets-get-a-6-or-more-gpus-supported-mobo-thread-going/