Mixing 2Rx4 and 2Rx8 Registered ECC memory -- will it work?

[Europa2010AD]

I have a TrueNAS Mini XL+, which uses the Supermicro A2SDi-H-TF motherboard. It has 4 DIMM slots and came with 2 x 16GB Registered ECC 2Rx8 2400MHz DDR4 memory.
I'd like to add 2 more 32GB memory to bring the total to 96GB; however for the life of me I just can't find any matching type that's in 2Rx8 -- the closest I can find is 2Rx4 made by Micron on Amazon.
Does anyone know if it's safe for me to mix these 2Rx4 and 2Rx8 memories?

 

[Etorix]

It could be that 32 GB modules are made as 2Rx4 rather than 2Rx8. I think I've mostly (only?) seen Rx8 in 16 GB and smaller.
Mixing 16 GB 2Rx8 in one channel and 32 GB 2Rx4 may work—you'd be mixing sizes as well as ranks. But the safest option would be to get four identical modules (second-hand 2400 MHz RDIMM should come cheap enough…) and recycle the two 16 GB in another build.