"Buffer overflow" basically means that the memory card isn't taking data as fast as the camera is generating it. In other words, your card is too slow.
I am surprised to see this during FHD recording-- that's weird. Still.
There are a few things you need to be aware of here.
First, in dual recording, it's essential that both cards can handle the data. Since the SD card is only taking proxies, you wouldn't expect a problem here, but then again you haven't told us the speed ratings of any of these cards, so who knows, Terms like "Extreme Plus" and "stunning amazeballs" mean absolutely nothing -- a lot of those terms were introduced when 3½ inch floppies were considered leading-edge.
Second, you need to know that the speed numbers printed on the card are COMPL|ETELY USELESS. Those numbers tell you the best-case speed of the card, and in many cases tat's the read speed. For video, the ONLY thing that matters is the worst-case sustained write speed, which can be hundreds of times slower. No kidding, seriously, hundreds of times.
For example, I have a ProGrade card which says "1700 MB/s" on its sticker. That card doesn't work in the R5C because it's way too slow; I get "Buffer overflow" errors with it. The card I'm actually using -- a different ProGrade card -- also says "1700 MB/s", but it's much faster, and works fine.
So you need to look at the card's independently tested speed rating. For SD cards, that's the "v" number, like v30, or v60, or v90. If it doesn't have a "v" rating, then you shouldn't use it for video.
For CF cards, the equivalent would be the "VPG" rating, like "VPG 400". But VPG is very poorly supported. Look on the card manufacturer's web site, to see if they at least tell you their claimed sustained write speed. Or look on Canon's R5C FAQ, to see if your card is recommended by them : https://www.canon.co.uk/support/consumer/products/cameras/eos-r/eos-r5-c.html?type=faq&search=1&sear...
Hope this helps.
