How could I use an SDXC to CF adapter to have dual SDXC cards in the EOS R5?

[p4pictures]

I’m not sure an adapter exists that takes an SD card and allows it to fit in a CFexpress slot. One of the problems is the big difference in speed, CFexpress is four or five times faster read / write than top quality SD cards. Also some functions require the performance of the CFexpress card to work. 

[Waddizzle]

You don’t.  The two media formats are not exactly cross compatible.  Use the correct type of card.

[Tronhard]

Hi Phil:

Just to add to the chorus...  I had a talk with a Canon tech some time ago about that and it's not viable. As my colleagues have alluded to the card types are not compatible - and it's far more than a simple physical difference.  They use different technologies and protocols to address the two card types.

The closest one can come would be some Sony and Nikon Z cameras that use of CF Express type A card slots that are able to take and use SD cards, but it hasn't really been taken up by others and has not gained traction.  CF A are even more expensive than Type B cards, last time I checked.
You might find this article of interest:
What is CFexpress Type A? Sony A7S III's new memory card explained | Digital Camera World

[Philnick]

As an R7 user tempted by the lower noise potential of the R5 I'll be staying put for a while. The increased weight of the R5 as well as the cost of CF cards and readers militate against it. I get great quality images from my R7 (I mainly use full-frame glass) - Photos.PhilOlenick.com - though I just got the new Sigma RF-S 18-50mm f2.8 (which on the R7 is similar to my EF 24-70mm f2.8L II on an R5). I've been a Canon shooter since I got an FT-QL in 1968, which I used with the FL 85mm f1.8. Switched to APS-C DSLRs around 2005. The R7 is so similar in size and weight to the FT that it feels like coming home.

[Waddizzle]

I believe that Sigma lens is an EF mount, not an RF mount.  It is not fully compatible with full frame bodies.

You should take a look at buying refurbished R6 mark II bodies from the Canon online store while they are still in stock.  They can run out quickly and remain out of stock for weeks or months.  

[Philnick · Accepted solution]

That Sigma is the first Canon-licensed 3rd party RF-S lens with auto-focus. It was already available in Sony, Fujifilm, and Leica APS-C versions. There are several more coming this fall: 10 -18mm f2.8 and a bunch of f1.4 primes.

Your suggestion of the R6 II makes sense: lighter and cheaper than the R5, with dual UHS SDXC and probably even better low light performance. R7 with Sigma for walk-around and reach, R6 II with 24-70mm for low-light.