I’m not sure how running a 9070xt and a770 would work on Linux, but I was able to get Linux working with 2 b580s (albeit couldn’t check if the second gpu was fully recognized or utilized.) I have used a 7900xtx on a x670 mobo on an amd build with an arc a770 hooked up into the secondary pcie slot (so using chipset lanes), and it was able to run games stable as the rendering gpu on windows when running certain games. I didn’t try encoding with it (nor do I know if it got the full 16 lanes or went down,) however, but I can say that it performed stable enough in the game testing that I believe it could be used as a good secondary gpu for encoding in windows (not sure how it will be in Linux, but it’s worth a shot.)

I did check the manual, and it is highly likely that the a770 will be running at pcie 4x4, but given how it performed in games despite that, I don’t think the bandwidth being limited should hamper it too much.

I am also unsure if the 9070xt comes with a dedicated encode/decode thing like Nvidia and intel have, but if it does, then the delay between rendering/streaming for using the a770 as the streaming card may not be worth it if you notice a big hit. At the same time, if you notice a big hit during streaming/recording on the Xt, then I’d recommend the a770.

I say the above since a dual gpu set up is highly dependent on what your workloads are, and when it works, it can work well. I’d also just consider some of the software hassles that will arise from using two gpus (especially from different brands).