I saw comments saying they'd be running that setup for years without issues somewhere
I'm sure that somewhere on the Internet, someone posted such a message somewhere.
Given the correct SATA controller with the correct SATA chipset and FreeBSD or Linux port multiplier support for that chipset which was 100% rock solid, this has a chance of running halfarse-well under limited circumstances. However, even assuming the right conditions, you'd be bottlenecking because the output is being multiplexed on a single SATA connection, which is going to cause it to perform very poorly. There is no fixing that.
This is why you don't want to listen to random people out on the Intarwebz, because often they couldn't figure out what the obvious issues are even if someone came along with a clue-bat and pounded clues into their head.
Worse, if you actually read the links I provided above, you'll find that even the fine folks at BackBlaze, who had significant financial incentive to "make it work," finally gave up on SATA port multipliers. The number of truly safe-to-use combinations is pretty low, and practical issues such as reliable sourcing of parts on an ongoing basis was difficult. This means that it is difficult, certainly not impossible, but definitely difficult, to find a SATA controller that isn't a knockoff AND actually supports PM's well, and then a PM that is compatible with it, AND works reliably. So it isn't that there is no possibility, it's just that it's really hard and you'd need a real good reason.
Around here, we have a bunch of resources that are provided by people who have experience with FreeNAS/TrueNAS, plus a whole community of people who are checking and doublechecking their handiwork, so multiple sets of eyeballs typically fall upon bad information, and the bad information doesn't survive the experience...
Now, I don't really know what your goal is here. Your typical SATA SSD can handle north of 500MBytes/sec read and write. Loading two of them onto a single SATA channel will lower this to around 200-225MBytes/sec, average, when both of them are active (you lose a little to the overhead caused by contention). If this is just for a boot device, it could be okay if you can find a compatible AHCI SATA controller that also supports PM. On the other hand, if you are just looking for dense ways to hook up M.2 SSD, there are products like the Silverstone SDP11 that allow you to hook up four M.2 SATA SSD's to a single SATA power lead, and it provides four SATA data connections that are compatible with HBA's, SATA AHCI ports, etc. We use a modified variant of these in servers here, one stacked on top of another with spacers, to get 8 SSD's stored in a 3.5" FH space.
