I have a smallish Mini-ITX system that has 6 3.5 in HDD bays and a PCIe slot with a PCIe to SSD card. This works for me and fits my life style needs (limited living space, no room for large towers).

 

I currently have 5 of my 6 HDD bays filled, and my SSD cache drive attached via this PCIe adapter card.

 

What I want to do is have a BTRFS Raid 1 Cache Pool without using that last 3.5" bay.

 

I have been eyeing this card StarTech.com 2x M.2 SSD Controller Card NGFF Card Adapter (PEX2M2) and wonder if it'll work the way I want by adding space for two m.2 SSD devices. Which I can then form a Cache Pool out of?

 

Any thoughts or concerns with this approach?

Following up on this.

 

I discovered recently that this card actually uses the same chipset as the card I am currently using, so I don't see why there would be problems.

 

I might go ahead and make this purchase soon.

That appears to be an AHCI card so it ought to work - NVMe won't yet.

Just following up.

 

Got this card and my two m.2 drives yesterday. Did a pretty quick swap and everything worked as expected, with no hassle.

I use one of these in one of my unRAID servers with good results. It's similar to your solution but uses a pair of mSATA SSDs.