the idea is that it would use two connections simultaneously, doubling the speeds overall without affecting the individual connections at all. It would be like a reverse Y splitter in that sense.
SATA is a serial connection, so with a Y splitter the controller on the SSD would send the same data to each of the MBs SATA-ports, no easy way to split the signal in 2. In theory it might be possible, if you make a special controller board, that has a SATA3-connection to the SSD, and 2 SATA2-connections to the MB and reports as a RAID-0-device, thus splitting the data between the two connections. But I don't think a device like that exists, and it's probably very hard to make one.
9 minutes ago, WildWestRonin said:
Duly noted, I'll probably be getting the same SSD either way honestly, because I think it may significantly outlast the main system itself, but it is true that the modern consumer HDDs using the magnetic shingles technology are significantly worse at that sort of thing than the older / enterprise tech that I will have.
SMR (shingled magnetic recording) HDDs seem to be mostly used in external HDDs these days, because the manufacturers realized they're not performing well enough to be in constant use like most internal HDDs. So even most consumer internal HDDs are now back to CMR, which performs just fine, just do a little bit of research on the specific model before you buy one.