Ok a bit of an update here. It appears that IF you set Slot 1 (with your fast UHS-II card) to record RAW and Slot 2 to record JPG (with your slower UHS-I card) there shouldn't be any or any significant slowing of write speed or at least not as big a hit as you'd think. Way back in 2017 someone tested this issue with the XT-2 and found that as long as you set slot 1 to record RAW to your fast UHS-II card and slot 2 to record JPEG to your "slower" UHS-I card which was 95 mb/s+ (they tend to top out around 170 mb/s and are super cheap now) the write speed would be identical in terms of when the camera paused burst images. In the case of the XT-2 basically I think after about 116 burst shots it would stop... no mater if the second card was a UHS-II or not because writing the much smaller JPGs didn't create the bottle neck first. Now if they reversed the cards and tried to record the RAWs to the UHS-I card and the JPGs to the fast card it hit a bottleneck at around 56 shots.
Basically it seems when splitting RAW/JPEG you can get away with using the slot 2 with a slower UHS-I card without impacting burst speed and more than if the cards were both UHS-II. For me this is acceptable because it allows me to use slot 2 as a "backup" of JPGs (a worthy tradeoff to save the money of another large capacity UHS-II card to get a true RAW backup there). I suppose with the XT-4 which has slightly larger RAW and JPEG files than the XT-2 this could have changed slightly but I doubt it with improved CPU performance etc. I think the other eye opener is that the XT-2 did 56 burst shots in a row before stopping even with the SLOWER card, so assuming you don't want to do high bit rate video all but the most diehard sports photographers have no real reason to buy a UHS-II as the burst performance is still pretty darn good.
1. Using slot 2 to record JPG with a UHS-I is a reasonable solution to saving money, creating a form of backup (although not a RAW backup) and without impacting burst performance.
2. If you don't intend to shoot high bit rate 4K video, it may be more than acceptable to use ALL UHS-I cards for most photographers as while burst rate drops quite a bit, it still is far more than most folks would find themselves requiring.
3. The fastest burst rate is ALWAYS if you shoot ONLY raw to a single card with NO recording of a backup or JPG to another card. Just 1 RAW to 1 UHS-II card. That's the fastest of all.
My final solution was to put a $145 128GB 300 mb/s UHS-II card in slot 1 for RAW (3900+ compressed lossless photos + video is dedicated to this slot) and a $30 170 mb/s UHS-I card in slot 2 for JPG (7700+ JPG photos).
Here is the link to the experiment: https://www.fujix-forum.com/threads/raw-jpeg-slower-card-in-second-slot.66119/